To put in new pistons, you have to pull the engine, buy a set of pistons w/ pins and rings, remove the engine, pull the intake and heads, pull the rods and pistons, have the block bored, honed and fit for the new pistons, have the old pistons removed from the rods, have the new pistons installed on the rods, reinstall the pistons, reassemble the top end, reinstall the engine. May need a balance job, also.

Heads and cam sounds a lot easier.