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How To Install a One-Piece Floorpan Replacement


One Piece Replacement Floorpan

Steve Baur

When it comes to rust repair on a classic car, you used to have only one option and that was to buy an individual replacement panel, cut out the old one, and weld in the new one. Thanks to Thoroughbred GT, there is another option for floor repair: the full one-piece floorpan.

You’ve probably seen full floorpan sides, and maybe even a full cabin floorpan at this point, but Thoroughbred GT takes it to another level by offering a full firewall to taillight panel floor that is fabricated in a precision jig to make sure that everything you build on top of it has a great foundation to work from.

Canfield, Ohio–based Thoroughbred GT offers its full floors for ’65-’70 Mustang hardtops and fastbacks, as well as ’65-’68 convertibles. The company also offers coupe to fastback conversions, full chassis skeletons, and “Clipster” back-halfs (everything from the firewall back). It also offers the skeletons with completed panel installation.

The 1969 SportsRoof Mustang that is the subject of this one-piece floor installation is a project car from our former sister magazine, Modified Mustangs Fords. If you caught part 1 of the Thoroughbred GT floor installation in the Nov. ’14 issue of Mustang Monthly last year, great. If you didn’t, you can find it online at
www.mustangandfords.com/how-to/paint-body/1411-how-to-install-a-one-piece-floorpan-replacement-part-1/. In that story, we covered the build process for the floorpan, as well as some initial deconstruction of the car. We started by gutting the car to the bare shell, and then drilled out the spot welds and extracted the seat pans. That would provide access to the inner rocker and the spot welds that attach it to the outer.

The subject vehicle had a bit of rust on the front floors, but more of a concern was the rear section of the car. It had sustained a rear passenger impact that wrinkled the trunk floor and bent the framerail upward 2 inches. The one-piece floor would fix all of that in one fell swoop. Anytime you weld on metal, the heat that is generated can warp, twist, and bend the metal that you are working with. We like that the one-piece floor is welded in a jig to maintain its shape—welding individual panels can lead to misaligned panels, especially for the novice DIY builder.

Removing the entire floor of your car is no small undertaking, and it shouldn’t be approached lightly. The floor actually adds a lot of rigidity to the chassis and removing it will weaken the overall structure of the body, so fabricating some sort of support inside the car is essential. While it is possible to perform the full-floor installation without the use of a chassis jig, special attention must be given to ensuring that the floor remains level and square as you marry it to the body. Measure and measure often!

If the one-piece floor installation is too big of a job for you to handle on your own, there are plenty of professional restorers out there who can tackle it. If you’re going to pay the money to have someone do it, look for a shop that has the jig to fit your car. We went to Gillis Performance Restorations (GPR) in Port Richey, Florida, where Rusty and Brian Gillis handled the majority of the Thoroughbred GT one-piece floorpan installation. We’ve worked with them in the past on a number of projects and tech articles, and GPR also has the chassis jig to make sure the floorpan is square form the outset.

Check out the photos and accompanying captions to see how the Thoroughbred GT one-piece full floorpan goes in. With a solid foundation like this, you’ll feel a lot better about building on top of it.

01. This is what’s left of the ’69 SportsRoof after removing the rear quarter-panels. In fact, the whole back end will be coming off, as there isn’t a straight or sound piece of metal back there. Take special note of the wrinkle in the trunk floor right behind the rear wheel house. The rear collision buckled the floor there, as well as the C-pillar support grid above it that is now exposed.

02. The rear portion of the floor was relatively clean, but the front sections where they meet the firewall had a number of issues, as seen here.

03. Before you go hog-wild cutting the floor out, now is the time to take measurements to check to see if it’s square and to have something to go off of once you put the new floor in. Gillis Performance Restoration’s Brian Gillis took measurements from the driver-side front leaf spring perch to the passenger-side front control arm mounting hole in the shock tower. He also took measurements from the front of the torque box to the doorjamb on the same side, as this will make sure the new floor is in the right position front to back. The seat pans have been removed, as the inner rockers need to be separated from the outers, and we need to get to the spot welds that the seat pans cover up to do that.

04. As removing the floor will substantially weaken the body structure, 3/4-inch square tubing was welded in to support the body and keep it square and rigid. It might look a bit like overkill now, but a couple of images from now, you’ll know why.

05. A combination of plasma cutter, cut-off wheel, and body saw was used to cut out the factory floorpan. Pay special attention when cutting the firewall section up front so you don’t leave yourself short.

06. Moving on to the trunk area, the rest of the floor has been cut away, as have the taillight panel and passenger-side wheel houses. This is where those support bars really come into play.

07. The Thoroughbred GT floorpan has the inner rockers built into it, so the old ones can be removed. Rust damage can often be found inside the rockers, but the ones on this vehicle were rather clean. Still, it was sanded down to bare metal and epoxy-primed to ensure it stays rust-free for another 40-plus years. If the outer rocker is damaged, you’ll need to make repairs to that once the floor is in, as the inner rocker provides the support.

08. It’s now time to marry the body and floor in glorious automotive bliss. Here you can see the chassis jig that secures the floor at its suspension pick up points.

09. When marrying the floor to the body, you’ll need to pry the rockers gently over top of the seatbelt bolt bosses in order for it to slide down. Other than that, it’s just a matter of lining things up and matching your previously taken measurements.

10. The inner wheelhouse on the driver side was still in good shape, so it was left in place, so that when the floor was lifted in we could see that it lined up with the wheelhouse properly. This is especially helpful if the wheelhouses are original. It’s always possible that aftermarket ones could have been installed incorrectly and out of position.

11-12. Always make sure that the jig, surface plate, or other build structure is level before you begin building on top of it. Here, Brian is checking to make sure the framerails are level from side to side, and that the rockers are level front to back.

13. With everything level and in the correct position, Brian drills out holes for sheetmetal screws that will secure the floor in place while he starts the plug-welding process.

14. This is looking down at the front of the driver-side wheelhouse, and as you can see, the floor lined up well in this multi-angled area—it meets up with the wheelhouse as well as the rocker without any bad gaps.

15. Overall, Brian felt the fit and finish of the Thoroughbred GT floor was very good. This shot is right behind the passenger door B-pillar and as you can see, the inner and outer wheelhouses have been excised since they were damaged from the collision.

16. Plug welding begins to permanently bond the floor to the body. It doesn’t hurt to verify your measurements during the welding process to double check that everything is still straight and square.

17. Brian decided that it was best to replace the firewall in the car. The original was removed prior to the floor installation, and here he his mocking it up to check fitment. There’s still a bit more rust repair to do to the inner cowl side panels, and new torque boxes are going in up front as well.

18. There’s no doubt that the finished floor is the most solid and straight piece of the car at this point, and we now have a great foundation to build on and bring this SportsRoof back to a sound state.

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