Can you post some pictures of what you have done so far. Like, I was saying, I believe you should be able to use any conventional doors from 1/2 ton thru 1 1/2 ton for the rear door. There was this type of conversion available back then when ordering a truck for crew work. GM had outside companies convert regular cab trucks into crew cabs. I've seen three door cab COE's used for Firetrucks, ect. many times.
