Leaf spring perch variations???

[HSRACER]

I have removed two rear differentials out of leaf spring 1970 2wd trucks. One 6 lug chevy and an 8 lug gmc. Both the perches on these diffs measure 42.5 inches center to center.

I have had a couple of people interested in purchasing these for different trucks. One had a 1965 chevy 4x4 and his perches measured 40.25 inches center to center so they wouldn't fit.

Another guy measured his 1971 4x4 blazer perches and they were also 40.25 inches center to center.

My question is were all 4x4 differentials set at the 40.25 inch setting and 2wd diffs set at 42.5 for our 67-72 trucks?

I could use an education!!! Thank in advance for your insight.