Re: Merc1978 80hp LU
Your lower unit will bolt up to any 66 C.I. Inline Four and any Inline Six from 'approx '66 thru the last production year for the Inline Six ('89 if I recall).
The 800-4 lower unit has a 2.3:1 gear ratio which may not be suitable for a 1500, but it would bolt up. The smaller size of the 2.3:1's pinion gear would probably not be an issue on lesser-HP motors.
Repropping may be necessary depending upon the gear ratio of the lower unit being replaced.
HTH...........ed
