I agree with stock49 especially as to custom pistons. Be careful with main bearings as they have different bores. There are 2 rod bearings. I think the only difference is the width of the tab that fits the notch in the rod. Thus there are wide and narrow notch rods. otherwise all GMC rods are the same. They are strong and good for all the power you are likely to make.
There are 4 different heads. This link tho Patrick's catalog shows the differences.
Patrick's I have been running the later 270 style head since 1978 and find it works fine for a warmed up street pickup. You only need the big port heads on an all out race motor. Some port work and bigger valves will do all you need otherwise. Bill Fisher did not recommend the big port heads for a mostly street engine. A supercharger will make the big ports unnecessary. It is a shame that no-one has written a GMC speed manual since the 50s. A lot has been learned since then and newer seals, valves, etc are available.
