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.