This past July I had the pleasure of introducing my daughter to manual transmissions in the exact same car I first learned 40 years earlier.
At 22 it was the first time she had operated a manual, and I'm willing to bet she has not driven one since. There may be a day when she needs that knowledge, but it does not any longer seem critical.
Given the year range you're looking at Bill I suspect many of those replaced transmissions are early 700r4s when they were still working out the design. The 5-speed used behind most Cummins has a fault that can loose you 5th gear. Manual transmissions might be more tolerant of wear but I'm not convinced there is an inherent service life advantage when comparing modern automatics against modern manuals as a whole.
My '72 has been taken out of service, not because it doesn't run, but because I don't need it and its too rusty to seriously consider restoring. It has 100,000 plus on its second motor but still has the original TH350 according to the son of the guy who bought the truck used in 1974.
I think its a personal preference thing and if you prefer manual transmissions I have no objection to you driving one. The older I get the more I want an automatic in mine.
