Bigger main diameter creates more over lap between the rod and main journals making the crank is stronger.That makes the cast crank a pretty decent piece. The stock heads are all pretty much a wash except for intake valve size. The stock 360 lacks compression and cam. However the magnum has the roller advantage and better head. I've run al of them. To be honest I really see no advantage with the 340 except the bore is a little bigger. The magnum blocks have far better machining from the factory. If its getting rebuilt the the pistons and cam are getting swapped anyway so who cares. As far as cast crank strenth? We make well over 800 with a factory 360 crank. I currently have a 340, 360, and a big block. Any body that thinks just because they have a 340 or a 360 and that's the difference if its quick or not makes me wonder.
Doug