You have your hands full for sure. Try to weld up bracing from under the dash(cowl area) down to whatever is solid near the seat area.If you think the door openings might splay add a cross brace on the jambs.Don't take out big sections all at once unless your are sure of your bracing. As far as commercial aftermarket body dealers for these trucks, I've rarely seen them at Carlisle,but have at Hershey. You may find a shadetree mechanic selling used cuts from trucks or a panel he doesn't need. Good Luck.

Drew