So, I take it that's not your transmission in the pictures? There is obviously a spacer between the bellhousing and the trans case, and the logical thing to do is to add a plate at right angles to it to bolt the pedal bracket onto. Them use angled braces to the front and rear over to the frame to reinforce the pedal mount. Those braces will need to have rubber vibration isolators to prevent transferring engine noise to the frame and cab.
David, the original pedal assembly was bolted to the bell housing, so there won't be any more movement of the pedals than it had when the truck was new.
Jerry
Correct, these are pictures of Cavebull's swap that I am using as a guide. I understand what you were saying now, the t5 adapter plate between the trans and bellhousing is another opportunity to add some stability. Not sure if it is aluminum or not but I bet it is...easier to machine. Looks like this problem doesn't have a standard solution, so I'll have to address it when I get there. Good idea with the addition of some rubber mounts between the bracket and the frame. That should add some more strength but also allow it to move just enough as well.
David, yes that's all that they had back on these trucks. The more I work on my art deco truck the more convinced I am that it really is just a tractor with a cab on top!