Bought a kicker... need a lower unit tho

[smokeonthewater]

sooo I went to look at a "'86 merc 7.5" today and bought it... I believe it's a 1975 model tho... The lower unit is munched but the engine runs great.
I gave $100 for it so I think I did pretty well.
Now on to the details.... My engine serial number is:
4132901

If I run the number it comes in as a 1975 merc 7.5 hp model 75......

How do I determine whether a certain lower will fit or not?
Looks like the part number on my lower is IIRC 1630 3916 and I found a 1630 4407 for $50