What year is the block? The 28 oz. crank fits into a 2 piece rear seal block. The 50 oz. crank fits into a 1 piece rear seal block. The problem is there are
oil slingers on some cranks at the rear seal area and if my memory is correct the one piece rear seal block does not have clearace for one. The 50 oz. crank has less mass in the counterweights and must have the 50oz. balancer and flywheel or it will not be close to being in balance.