I have a '57 270in a '53 Chevy pickup (5 window) I originally built it in 1978 I think. So I have the same block & head as you. It seems that the older style (domed) pistons are easier to find now. They sometimes show up on eBay. That would require you to use and earlier domed chamber head.

There were only 4 different heads used for all the GMCs 228-302. #1 had dome chambers and small ports, #2 (H-head) had dome chambers and big ports. #3 was the later 302 with a D shaped chamber and big ports. #4 the ones we have for later 270s with a sorta D chamber and small ports. For a street engine small ports are fine. Bill Fisher recommended small ports for the street.

The rods for all of these engines were the same length and had the same bottom end bore. There was a difference in the size of the notch that held the main bearing insert. There were two piston pin sizes.

The 302 crank should be OK. I'm not sure how many flywheel bolts it has. Early cranks had 4 and racers sometimes drilled 4 more. Later cranks had 6 bolts. This obviously effects flywheel choice. This can be an issue because 6v & 12V starters use a different number teeth in the ring gear. You can run a 6v starter on 12v.

Don't give up! These are good engines and fun to drive. I just got mine back in the road today. I had a fuel pump issue and just stuck an electric pump on it. It has a T5 & Dana 44 limited slip with 3.55 gears. Fenton headers a 4bbl carb and custom ground cam. It will cruse 75 mph all day and get 17+mpg.