Rex - the approach you describe sounds similar to what I'm trying to achieve. In the front section, between the "frame rail" and the rocker panel, there is 1/8" thick steel sheet already in place. The foot-plates for the cage are 1/4", as required by the rules. the fit at those locations is pretty good between the 1/4" and the 1/8" steel (both surfaces are flat), and there is a lot of reinforcement in that area. I agree with your analysis, it's not a good idea to weld 1/4" directly to the 0.060" if possible.
I hadn't previously considered the possibility of using 2 thicknesses of 1/8" steel to achieve the 1/4" overall thickness however.
At the rear section (intersection of rear floor and inner "cab corners", I have repaired the previous rust with overlapping sections of 0.062" thick pieces). My intention is to weld the 1/4" foot plates onto the cage first, and use 1/8" thick material to tie the 1/4" to the 0.062" material in this rear area, where the curvature is more difficult to match.
With a little bit of torch and hammer work, the 1/8" steel is fairly easy to beat back into the corners of the truck cab structure, as you have described.
Even though the existing record for the class I'm chasing is under 110 mph, I have dreams of running even faster in years to come. Safety First!!!
