WOW, you aren't starting with much there! I don't have the measurements you're after but if I were were you I think I'd research buying a replacement lower panel to establish the rear body width or you could also just find or buy a replacement decklid to establish the rear body width. Once you know where the quarters need to be spreadwise then you can cross measure the top in several places throughtout the length of the body and also cross measure the floor area to keep everything is squared up then start making the parts that are missing...hope that made sense. Work the project directly on the floor if it's flat and level or consider building a temporary table, trying to do it on a frame is going to be a PITA. Once you've got all the sheetmetal squared up, braced, and fitted then move it to the frame and construct the floor. Just some suggestions.
I've got the rearend of a 29modelA coupe and the front of a late 20's Whippet that I've been thinking about putting together- I did a quick cut and mocked them up together and the lines actually come real close to perfectly aligning with better proportions than a stock A coupe, metal condition is just a bit better than yours judging from your pics. All I need is more time, someday....
Good luck with the project, hang in there cause it's going to be a long road. Bob