If you are just looking for stock replacement pistons then get military surplus ones. Both the 270 and 302 were in military vehicles and parts are around. However, if you are building it for performance then either Egge or Veniola will have pistons. It won't be cheap.
I'm pretty sure the 340 GMC is a 302 bored out to max and also stroked by offset grinding the crank. Not for the faint of heart or wallet.
