I would think that the measurements would be off slightly so at the least you would probably need a slightly longer belt. The distance from the pulley that you mount on the deck spindle to drive the power flow unit will be slightly further from the chute opening on the side of the deck. The blades are a little longer than 1/3 of the total cutting width for overlap, so a blade for a 48" deck is a little over 16" long, and on a 54" deck it is a little over 18" long. So the difference in distance from the center of the pulley to the edge of the deck is going to be about half the difference in length between the two blade lengths, or about 1" further on the 54" deck than on the 48" deck.