C-10,
It's not the the engine is unusual, but rather overly built. The stock pistons were nicknamed Coffee Cans for a reason.
http://inlinethumb50.webshots.com/45...500x500Q85.jpg
MY son when 16 and we were taking the first engine down.
The stock bore was 4.25" and the pistons you can get from Kanter or Egg . and come 4.25 + .20, .40. & .60 over size. Remember these are 4 ring pistons 3 compression and 1 oiler.
The skin on these piston shafts are something like 1/4 thick or more and the way the engine is cast is that 90% of the sleeve is exposed to the water jacket, that why she ran so cool.
All the information you ever WANT to know is here.
http://www.6066gmcguy.org/gmcv6a.htm