found this..."Noticeable differences are (1) Thrust bearing pad on the side of the 1979 crank is bigger than on the 1968.(2) the counter weight part of the crank is much larger on the 1968 crank verses the 1979 crank and the counter weights are not in the same position on both cranks.Cranks fit into the block with no issues "
Thanks very much! I see that Crankshaft Supply shows the same 318 crank from '76 though the Magnums. They show the heavier for '71-'91; not sure what to make of that.
But the main question has been answered... why the PN change.
