If I'm removing the floor & gutter area, I weld a piece across the opening on the inside of the pinchweld, just below the "bulge" area on the door (hidden by the '77-up trim panel, not on the '73-6), this way you can close the door to check fitment. As Blazin' mentioned, the cowl is often back/down if the floor area is weak. A port-a-power or similar jacking tool may be needed, then install the brace.
As allways...Start by mounting the door your going to use along with good hinges before you start to weld. Lorne