I still think you should scrap that shell and replace it, but hey, I've been where you are right now :thumbup: I fixed mine enough to keep it together, sleeved 3 feet of the rear right frame rail with 3x6x1/4" box, replaced all the floors from the bench seat back with 16ga sheet, completely replaced the rear shock crossmember with 2x2x1/4 box tubing (welded to a fish plate on the frame rail I didn't sleeve), rebuilt the gas tank supports, etc. It was a great learning experience and I could probably finish the rest of the rig and make it as good as new, but never again, just not worth the effort. One of those things I'm glad I did but will not repeat.
I just hate sheetmetal, especially rusty sheetmetal. Welding to it is extremely frustrating, I hate having a bead going just fine and then suddenly blow through because I don't realize the metal went from normal to paper thin due to a rust spot on the other side.
Keep an eye on things as you're welding those mounts on... the frame will sometimes look fine on the outside but be rotten on the inside, and you'll blow through it like butter. If that happens, use fish plates across the spot where you blew through - thinner ones, say 1/8", with plenty of rosette welds. Make sure you turn the heat down from what you had it at for the 1/4" (don't ask :gag

if you end up doing this, and try to avoid vertical welds, stick with rosettes/plugs and horizontal welds when possible.