Any experiences with putting a 'modern' (one with an overdrive would be very nice) tranny behind my 402, gas sucking pig? That might make my truck not as 'for sale'.

My questions are
1) Flywheel
2) Clutch (Hydraulic or conventional?)
3) Bellhousing (I'm assuming it would bolt up to the block as there are no differences in bolt patterns that I know about)
4) adapter to a 205 (or should I locate a 208, 241 or whatever came with the trans?)

The flywheel shouldn't matter. It's matched to the engine.

The NV4500 takes an 11" or 12" clutch. Go with the 12".

The NV4500's used a hydraulic slave cylinder internal to the bellhousing. I'm using an external slave cylinder with the AA bellhousing. It would be very straight forward to use the mechanical setup with the AA system since the fork is a GM part.

Stock GM bellhousing that came with the NV4500 would bolt right up. One thing to note is that GM used 2 different bolt patterns for the NV4500 so make sure you get the right bellhousing.

The 241's are pretty good T cases unless you bottom out your front driveshaft or bang it on a rock. The NV4500 from a Chevy uses a 32 spline output shaft. I'm not positive but it's possible you could bolt a 205 from a TH400 to the back of it without an adapter. You'd have to check. I wouldn't even consider a 208.