[SCOTI]

Originally Posted by 66-PMD-GMC

Initial findings are as follows;
Wheel centerline datum holes, marked by red arrows, on both the 69 and the 63 frame are in identical positions on both frames.
Core support brackets 1.286, marked with black arrows are in the same position on both frames.
Core support/ cross member, marked by yellow arrow is not the same on both frames but frame rails are the same distance and swapping in the correct rail from the donor frame is a simple matter of using clamps, measuring and drilling holes.
Rear cab mounts are as SCOTI mentioned, not close at all between the frames, marked in green arrows. Again, clamped in at the correct height and position they seem relatively easy to install.
I did not get a chance to cut off the correct frame horns and mock them up to measure but I will soon.

Hhmm..... When I measured the core support mount (brkt 1.286), they were @ different heights vs the top of the frame rail when measured from the wheel centerline datum holes as well as a different distance (again measured from the wheel centerline datum holes). It wasn't off by much, but it was different.

If your data is correct, I'm glad I made them a bolt-on.