Ralph and group, I plan on using 3 70' Clerestory roofs from Bethlehem. They will need to be spliced to get the 71'6" +/-, the measurement for the Pennsy 74' cars. I used the Bethlehem B70 roof to determine this.
For the M70A, I have 3 Rivarossi 72' Heavyweight RPO's. The reason for them is the windows. One side has 3+space+3 windows, while the other side has space+2+space+4 windows. The M70As were built by ACF with different sheathing than the M70 as per the Keystone article. The Rivarossi RPO has only 3 windows per side.
I plan on cutting up the 3 cars and make flat kits. I will have to rely on diagrams and other data to determine proper spacing. I am thinking of scanning the drawing in the Keystone and shrinking it to HO scale.
I was originally going to reuse the Rivarossi roofs, but no dice. They are too short and too shallow, therefore leaving a large gap between the car ends and roof, about a scale 8"!
BTW, I did not measure the windows between the 2 cars. The M70B windows appear larger, just saying. I will measure again when I return to the project.