My lower end is toast after friends tried to use it to break submerged rocks.
my boat is a 1991 Arriva w/350
I have no idea how to determine gear ratio while finding a replacement.
Also, recommendations for best place to find a replacement for the best price would be very much appreciated

On the decal on the l/h side of the upper unit there will be small #’s on the decal like 1.47R

This indicates a 1.47:1 drive and from what you describe for power, it’s probably what you have.

Ayuh,..... SEI sells aftermarket drives, with a 3 years no-fault warrantee,......

thank you!