Hey Bishop_68.
I faced very similar questions but had a different starting point. I started with a N/A stroker 390 that should be in the 600hp+ range. Yet to be built.
I looked at the cost to build an AOD or 4R70 but went with a stock 6R80 as they can handle that HP/TQ as is. The 6R80 could be had for a very reasonable price back then. My cost was 1,000 per gear all said and done. Not a budget build but that’s the price for wading into new territory. Check out my build and you will get an idea of all the little things that are needed to get and electronic transmission fully functional in a 68 Cougar.