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This is going to be in two posts as I have 8 pics to upload, so 4 on this post and 4 more on the second post.

Many ways to box/stiffen the side rails of an open frame. Convertibles used a boxed frame and it seems some GTO's were fitted with the HD frame option which appears to be the convertible boxed frame. There are a number of kits available from basic to high tech and inexpensive to expensive. I am cheap, so went with what I feel is an inexpensive way to go and because I can do my own work. You could also fabricate flat steel plates to do a similar thing, but I felt the kit was easier because it is pre-cut.

I degreased/prepped the bare steel sides that were to go inside the frame rails with High Heat barbaque black paint. At 1200 degrees, I felt the paint burned off would be minimal. I followed this up after all welded with Eastwoods Internal Frame Paint. Eastwood Internal Frame Coating 14oz Aerosol

The ABC kit Frame Box-In Kit 2pc. # ABC-6872-2 Frame Box-In Kit 2pc. # ABC-6872-2 - $145.00 : ABCPerformance has a front and rear, so make sure you position it correctly.

I had to straighten my frame rail flanges as past jacking up of the car using the rail flanges bent them upward. Got them fairly straight on both sides. Instead of welding the plates to the outside of the frame as most would do, I fitted the plates about 5/16" inside the frame rails to create an "I-Beam" of sorts which I felt was stronger than simply boxing the frame and relying on the welds to take the twisting and forces applied to the side rails. Also figured it would make a neat space to relocate/protect my fuel/brake lines. I had to reshape the center tab to fit under the center body mount located under the door. Some say this is either not there or simply a rubber support and no bolt - that the convertible frame had the extra and bolt on frame/body mount. Mine was a bolt-thru frame/body mount, so maybe that was changed in 1968?. Used PST polygraphite frame bushing throughout.
 

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I got the flat steel plates in position. At the top you will see where is looks to be on the outside of the rail and not inside like I said. This is correct, it is on the outside as of this picture. I got the bottom of the plate fitted/clamped where I wanted it. Then I have the center tab at the body mount positioned under/inside the frame rail where I wanted it (had to keep grinding/fitting this tab to get it right where I wanted it). Then I tack welded the bottom and the tab at the upper frame.

Once the bottom and top tab were spotted in place, I was then able to get a screw driver in between the top rail and flat plate and pry the flat plate under the top rail flange. This is why I tacked the top tab in place so as to hold the flat plate outward and in position. When I pried the flat plate into position, it was forced/pressed up against the inside of the top rail flange with no big gaps needing filling - it was basically butted nice and tight and the added benefit was that it corrected some of the slightly bowed side rail flanges at the bottom.

You can see the flat plate welded in place and how I set it inside the frame rail. I tacked the plate in small tacks and spaced the tacks far apart so as to not over heat the frame side rails and wind up warping something. The body must also be bolted down to the frame at this point to keep the frame rails in their correct position. I would NOT do this without the body securely bolted down in place. Then I came back and added longer welds, but did not weld the plates solid, just about 2" welds throughout. A solid weld is not needed because of the "I-Beam" configuration. If I had put the plates on the outside edge of the frame rails, I would have needed to weld everything up solid to keep it from moving up/down under flex conditions- like lifting the front end off the ground 3 feet LOL. I also welded the ends of the plates to the front/rear frame sections to tie them in.

The last pic shows the plate/frame cleaned up and painted. DO not undercoat at this time unless you are completely done with any additional welding. Undercoating will catch fire easily. I still have to fabricate my transmission crossmember mounts, but since I am using a TKO 5-speed, it places the mounts a little differently and I have not yet fitted the engine trans in place to get the mounts into position. I plan on modifying my original trans crossmember and either welding or bolting a mount on to the boxing plates - just have not gotten that far yet.

So this is what I did on my '68 Lemans and just gives you an idea of what can be done. Can't say what I did is good or bad, but in my mind I feel the "I-Beam" concept makes for a stiffer frame over the standard open frame side rail.
 

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Agree. The frame should be much more rigid compared to before.

Probably a little late to be of help....here are some photos of my 68 convertible frame in case you are curious how those look. There are 3 different type of body mount cushions. All hockey puck style....two with holes and the third without. On the two with holes, there is a standard size (qty 8), a slightly smaller size (qty 4) and qty 2 of the type without holes. The small sizes are located up front at the firewall toe boards....1 pair per side for a total qty of 4 cushions. All 4 are nut and bolted down. The one under the door has a bolt as does the next one back under the rear footwells. The next mount back is where the solid cushion goes (without any fasteners).....this is the one just ahead of the coil spring pocket. The last two locations are standard size with bolts.

I'll also include some photos of the brake line routing in case that helps you any. The convertible brake/fuel lines are different between convertible and hardtop but your lines should still work fine with the boxing method you used.

Looks like nice work you did there! :thumbsup:
 

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I'm with Shake-N-Bake, your I-beam concept should definitely be stronger than the usual way this is done. Nice work, Jim!

Shake-N-Bake, your frame work is beautiful.

My '68 frame I'm afraid is just factory original paint (but not bad as it is a California desert car).
 

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I'm with Shake-N-Bake, your I-beam concept should definitely be stronger than the usual way this is done. Nice work, Jim!

Shake-N-Bake, your frame work is beautiful.

My '68 frame I'm afraid is just factory original paint (but not bad as it is a California desert car).
Thank you for the compliment. That is cool that your frame still sports the factory finish. Original finishes that are still presentable are great for the hobby.

Unfortunately, the finish on my frame was quite rough so it had to be redone. Probably over done to be honest but will likely be par for the course on this particular restoration. Sadly, I have a feeling this car might not get driven much when it's finished due to the rarity of the vehicle. Could be wrong though....that would be great. :)

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