IF you go big block, you'll need to call Moser sooner rather than later. If you go small block, you'll be okay for a bit.
As I'm looking at which powertrain to go with in my Buick, I've found that it will cost me around $1400 in parts just to rebuild the Nailhead, but I can get an under 100k mile 6.0 with a transmission for $2800. Yeah, I'll have fuel pump issues to deal with, but I think in the long run, I'll be happier with the LS engine.
Sweet score on that GP, btw. That distinctive 1980 model grille won't be mistaken for anything else on the road.