Have no fear, rear seat tube is shitty seamed tube as well. I may or may not have flattened the underside of the tube a tiiiny bit where it was hitting the frame rails to make it sit flat. 1.5" tube magically becomes 1.375" tube for two 3" spans.
Finished lowering the rear of the cab floor, which leaves the frame rails poking into the cab a bit. Welded up the remaining random holes in the cab as well. Cab floor is a patchwork mess. I definitely see why people just cut the floor out entirely and fill it back in later.
Just need to get the cab to a-pillar gussets under the dash in place, and I think I'll be pretty much done with the cab metalwork.