Sonic Blue, the cranks are the same but you should make sure the bob weights are the same. If they are then it will not cause any vibration, but I would double check it.
of course its balanced differently for the heavier rods.
but if you're having your reciprocating assembly balanced anyway (eg., rebuilding/overhauling) then its a non-issue, except the cost of the mallory metal which should be negligible anyway. only time i heard of people being charged for mallory metal was when they were converting an externally balanced crankshaft to internal balance (eg., 428 ford)
that is the only difference...but it's fully-counterweighted and I believe our blocks have additional clearance in the center bulkhead to accomodate that