Re: force prop interchage with a 72 merc
Splines are correct and prop length is correct BUT: the hub diameter may be too large. SOME Merc props will fit Force gearcases: IF it is a small (I think 4 1/4 inch) diameter gearcase then the prop will fit if it is the larger gearcase, the prop will not fiy However, I think you are out of luck; I think the Merc 90s were large diameter gearcases.
IF it is a Vortex prop, then you are in luck! They have a plastic spacer/sealer ring around the hub. Pop off this ring and the prop will fit. No modifications, same thrust washer and hub kit.