dunno bout weight, but the df150 is a bigger block than the df140. if not one of the bigger blocks in a 150 4s. prob no matter on a 18 footer. but having a bigger block is better. again doubt it matters on a 18 footer. but knowing someone whose blown a df140 or 2, on a 21 footer non sc, he is pushing the engine hard. he wishes he had a df150
.