Getting the measurements for a 72 extended build

[WM72 custom]

I've been reading post on this side for years and i have decided to post a question I've been trying to figure out for quite awhile. I have had 70-72 chevy on a off for years and always wanted to build a extended cab. I had a good friend that built one about 10 ago, but he pasted away several years ago. I know how I want it (suburban front door and cut down single cab doors for the extended cab suicide doors). But non of the builds I read up on post how they measured for the cab cuts? Can anyone post any information on how they made there cuts. Im looking for a foot and a half to two foot rear door.