I have 2 spare lower SSM #3 drives for my current boat. One is a LH with a bad case and good gears and one is RH with good everything. I actually need a good LH lower to replace one on my boat, so what do I need to change out of the RH to make it a LH?
I've rebuilt just about anything with gears and used to set up automotive gear cases all the time (Dana 60s, 44s etc). I am mechanically inclined and I'm just curious which gears get swapped over and how big of a job it is.

Any help is much appreciated,

the prop shaft and lower gears will need to be changed to change the rotation of the drive......this is minimum...there may be more.

I suggest you get a service manual for that outdrive....there are many special tools needed (or make them like I intend) to complete the job. The manuals are no longer in print....I got mine used off ebay.

If you decide to do the change I have new #3 LH and RH lower gear sets for the #3 drive as well as new BIG propshafts for both.

The SSM III & SSM IV use the same propshaft for either rotation. The LH is a 4 gear set & RH is 3 gear set. The RH propshaft gear has teeth on both sides. The IIIA & V drives use different propshafts for rotation. With the parts being as expensive as they are & the special tools required I would highly recommend not attempting this yourself.