I removed and replaced all of the rear inner tubs on my 1963 Plymouth Belvedere a long time ago, I bought and used a rea tub kit from Alston's Chassis works and made it fit
I'm not a good fabricator or welder, I used pop rivets where needed and seal the upper tub to the 1.4 panels with seam sealer, I think it was some sort of seam sealer
The inner upright had a pinch seam that the upper tub fit into and pushed it together, I think it was called a "Pittsburgh" seam.
I ended up using duct tape on the inside of the upper and outer seams to keep smoke out from the burnouts. That worked