Curious as to what differences = other than re-proping a change from a V-8 upper gearcase to one from a v-6 wouyld makein running?? different gear ratios.in cases.
I think NW Redneck got all of the gear ratios collected in THIS topic. Not sure what the effect of having the wrong gear ratio on the upper would be, but most likely you're better served keeping the one ya' got.
Like Bondo said, play with the prop pitch. I've seen 'em from 13" up to 21".