72 SB 4x4

[Diesel doctor]

My son and I are in the process of rebuilding (ground up) a 72 4x4 pu. We have a rolling frame, now working on the cab. cab has moderate amount of rust, floor corners, front and rear cab braces (ends only), cab pillars, both front and rear (bottoms of both) and of course both inner and outer rocker panels. My question is, without the existing metal to get good measurements from how does one calculate just exactly where everything should go? I understand the outer rocker and fitting to cab, door, etc. Don't have any good experience with this year model truck. First time post. Any help would be appreciated.