Jonathan,
I don't have a HUP to help out with the measurements, but I'm sure you will find the help you need here. I can tell you the windscreen frame is detachable and the same as on other Chevrolet CMPs (quite possibly Ford too). It might be worth looking for CMP body parts as a man with your skills would be able to turn a couple of cab doors into a HUP body door?!? You can use the double curvature back section of a regular CMP Cab 13 roof as the rear roof section on your HUP body.
I must commend you for taking up this challenge. Quite a few HUP bodies were cut down and reconstructed later, but to date I have not seen a project which also includes reversal of a chassis extension plus engine transplant.
Hanno