Just like Walt I can see no advantage to putting the older design engine in the 81. You might consider going to a late 292 for a few more inches but you could also do plenty to the 250.
I haven't checked adapters but I think you would be hard pressed to find one to hook that trans to the 261 block.
The 261 would be great in an older Chev pickup or car though. I've had friends run them in the 47/53
pickups and really scoot with a few mods.
If the 250 is in decent shape or good enough to rebuild I think a four barrel or dual carb intake, hedders and a proper cam would be interesting.
