Where to start with body Install and rust repair

OK so last week we moved the body of my 57 onto the "new C4 frame". At the time we installed new polyurethane bushings and new body mount hardware, but it is all just on finger tight. I need to do some rust repair in the outer rockers, floors, and trunk pan. Then I will be replacing the dash and doing a recessed firewall. My thought is I need to get the body bolted down and the door and fender gaps as good as possible before I start this process. I have seen some shimming guides on door gaps as well as guides on setting front fender cowl gaps, but these are on cars that are assembled. Where is the best place to start? OR do you just pick a place to start and keep working around until it gets as good as possible? I am sure that this will be a process but just thought there might be some insight from those who have done this before. Thanks Mike

I don't know that I'd worry about the fender gaps in order to do the floors and rockers, but you will need to check those gaps eventually. You want to get the doors gaps as good as you can get them. Shimming will get you close, but to really get good gaps you're probably going to need to grind and weld a few places. You need to decide if you want the rocker and fender to meet, or if you want a gap there too. Mine are really close.

I thought I might spend some time shimming before body work, especially cutting out the dash and firewall, that way I dont weld in a twist that I cant shim out. I know welding and ginding are needed for good gaps I just thought getting it close first would reduce that to a minimum.