grid and brace.
I did a 29 nash a while back. I used a piece of 3/4 plywood screwed down on a big pallet and had it up on the body cart so it was easier to work on with out bending over. I drew a grid on the plywood to get every thing square and screwed down . I used pieces of scrap conduit to cross and x brace everything. My son is and elect contractor, and scrap is free. brace similar to the picts showing guys chopping a top.