Re: mercury 80 hp
Unfortunately there is no such thing as a '72 70hp. If you're thinking of a later-70's model 3-cyl 70hp, no that won't work. Totally different motor. Merc also brought out a line of larger-cubic-inch 3-cyl motors in the later 80's (70, 80, 90hp) and lower units from these will not fit your motor, either.
4-cyl 66 cubic inch motors in the 65hp, 75hp, 80hp, and 85hp flavor will fit your motor with minor differences such as O-ring groove or lack thereof at the driveshaft splines. In your case, a '79 80hp 4-cyl won't have a driveshaft O-ring groove 'cause the O-ring carrier is pressed into the end of the crankshaft. If you were to pick up and older L/U with groove, just fill it in with Bondo, JB, Epoxy, etc and sand flush.
Early 65hp motors won't have the 2.3:1 gear ratio you'll want for your motor. The 75, 80, and 85 models do. If you do happen to find a 2:1 gearbox, you'll need to reduce recommended prop pitch by at least 2".
Most Inline Sixes from '66-on will have a 2:1 gearbox, same issues as above. Early-70's 115's and 135's may have a different 'overdrive' gear ratio, best stay away from those as although they will fit, your prop choices would be very limited.
Best lower units are those mid-70's-and-up with stainless driveshaft (or stainless around the water pump/oil seals). These won't rot away like the older mild steel shafts, and cause oil leaks at the pump seals.
HTH, if you find a lower unit you think might work, post a link or pic and we'll talk.........ed
