We are VERY exacting on panel replacement and gaps since a botched job costs us serious money where the hobbyist is out sweat and labor. On total floor replacement we fit the doors, with no hinges, into the body and shim till they fit the opening. The doors are then welded to the body with 4-6 good tacks on the front edge and rear edge. Bracing installed at this point so doors can be removed and floor/rockers/pillars/cross members can be welded in. Pictured is a 37 Chevy Business Coupe that the entire floor AND firewall was replaced (firewall to accommodate a big block engine).
Bracing installed but not in way of door fitting hole:
Doors shimmed for fit before welding:
All gaps MUST be thickness of a paint stir stick:
Body molding lines and door gap:
New floor and firewall: