fiber vs sintered clutches - best ATF for both?

You should be OK with the ATF. GM has called every Dexron backwards compatible, so there should be nothing in it that would attack clutches. Theoretically, Dexron VI would work and give you a significant increase in viscosity index.

Unlike the NV 4500, which specc'd a specific lube for the carbon fiber synchros. As I remember, we didn't make an suitable equivalent, so we just said to use Mopar/GM/OEM lube.

It's funny, I used to drive by the New Process-New Venture plant near Syracuse several times a month for work. The place closed when the union voted to give up their jobs rather than to take a pay hike. I know it sounds crazy, but the plant had been losing money a long time. The wage concessions were supposed to help save it. The workers didn't believe it could be saved, so their vote reflected the fact that their unemployment benefits would be based on the higher wage.