Got a body assembly question.

Fall will be here before ya' know it and hopefully time to work on the project will follow.

I have the "Z" stripped to the shell. Since it was formerly a race car it has NOTHING from the back of the seat area to the tail lights. I am replacing everything but the firewall shell and the top.
My plan was to brace the old shell from the inside ( Kinda' like when ya' chop a top) so I can remove the top and firewall and keep the old shell for measurements to get the new shell assembled correctly.
Or
Get a donor top N' Firewall from a salvage yard so I can keep the old shell intact for measurements.
Or
Maybe just find a camaro or firebird @ the salvage yard and use it as a pattern. If I trash the top and firewall on mine all that will be left of the original is the vin. tag. With the car on a jig, I figured I can mount temporary brackets off it to hold rocker panels, quarter panels, etc. in place til I can fabricate permanent outriggers off the frame for the body mounts.
Or
should I replace one body panel at a time on the existing shell until I have all the parts replaced.

I guess as long as I get the body the right width and keep the door opening measurement right I should be O.K. ?? Maybe I;m just thinkin' about it tooooo much.