[Sheepdip]

I just recently did a front disc brake conversion on my 70-1 ton dually using a 86 Cab and Chassis donor, so I can't speak for a 72.

I chose to keep my 70 cross member as I saw no benefit to changing it out.
For some reason everything was identical dimension wise except the steering cross links, the 86 was about 1/2" longer difference at the tie rod tapers and 1" longer difference at the pitman/idler tapers.

I reused the existing 70 center link to a perfect fit and alignment. I did not have the 86 donor to measure why the difference but my thinking was the steering box/idler bracket spread.
Not sure if that's the area to look at, but worth a look.