Re: 502 crate swap questions
Regarding the flywheel, the 396 is an internal balance, 502 is an external balance. So yes, you'd need a different flywheel.
You would need a new pan. The 396 (Gen IV) pan won't work, and the pan that comes on the 502 is a 6 quart truck pan that probably won't fit a 1st gen F-body.
Looking at the curves that GM published for the 502/502, it's hard to define streetable. The 502 HP peak is above 5,000 RPM, but it looks like 550 ft-lbs starts down low (curve starts at 2,750 RPM, so it's hard to say where the torque actually starts).
The 454 HO (425 HP/500 ft-lb) might be a good choice if you have to have a crate; it can still use the mechanical style fuel pump.
Seems like a lot of cash. I would (and did) have the 396 rebuilt.
69 'vert project big block/TKO 600 RR
68 'vert driver RS clone -- gone!