If memory serves correctly and I could be wrong their square body frame was pretty bad that the donor parts came from so using the old C30 frame made sense. Just as strong and the body all ready matches the frame. My research is that the Cab & Chassis frames are very similar in many, many ways and not so much in a few others.
I really like this build also and have committed
to building a similar truck (see link). We have gathered parts to go either way but using a 84 GMC K3500 Crew Cab frame is making more sense in our situation for now.