you mentioned streetable cylinder heads----has that part changed???
If you just want to shed some weight, change the water pump and starter and manifold

More weight, change the heads to aluminum

If you just want pistons you will need to measure the bore to see what size 4.250 was stock, if it was ever rebuilt they will be larger, if it hasn't been rebuilt, it probable needs to go .020 or .030 over

the truck pistons aren't as light as the car pistons available but don't look for weight savings in pistons unless you are going to use a shorter piston design

Years ago, Ed Donovan used the truck heads as a core for some high performance stuff by rolling back the chamber wall to sort of a open chamber design even before GM was doing that.

Is this in your 81 c10?