1. A junkyard driveshaft is one option. Another option is an adapter flange yoke, Sonnax part number T35-ALFY-08. This gets you to a standard 1350 U-joint. From there, it's as easy as making a slip-joint driveshaft similar to how one would replace a ball & trunion joint (Early Valiant Barracuda Club - ball and trunion U-joint conversion)
2. The 8HP70 torque converter does not have an integrated ring gear. instead, there is an intermediate flexplate that goes between the crankshaft hub and the torque converter. By my measurements, the fix looks simple enough; just get a standard stick-shift flywheel for an LA motor, drill 6 holes in it, and bolt it to the 8HP70 torque converter. Picture attached of the torque converter. The 6 outer holes are threaded and would allow for a flywheel to be bolted to it.
3. Not sure on this one as my plan had this 8HP70 destined for a 3,740 lbs B-body.
4. You are correct. The tight gear ratios of the 8-speed would mean that you'll always be in the right gear at the right time. And you're able to jump on the laptop at any time to fine-tune shift points, firmness, etc.