The G50s and the 996's share some architecture. The front bellhousing face and the bolt pattern are the same. They
can use the same clutch system as well. The distance between the output flanges transversely is the same and I believe that many of the gears are the same as well as the internal shaft diameters so there is some gear interchangeability. There are a lot of gear ratios choices. Ask about this if you are interested.
The main thing that is different between the 996 turbo 6 speed and the G50 01-03 I took out of my SLC is the ring and pinion gears diameters. The G50 is 7.5 inches and the 996 is 9.
Here's my opinion. Its free. I only know what I think I know so......based on my limited experience......... The G50 I had was fine with 425HP and a Quaife TBD running on big fat slicks and only run on track. I added more than a 100HP to 550ish and more importantly the new engine makes a LOT MORE torque. More than 400 at 2800 and 550+ at 6000rpms. The little G50 R&P could not stand it and it failed.
If you are going to have less than 350 Ft/Lbs of torque, run the car on street tires and on the street and you drive like it's your money, then the small R&P G50s are a fine gearbox. Otherwise you need to get a Porsche with the big 9 inch R&P. That's my story and I'm sticking to it.
DO NOT buy a used gearbox on ebay and then take it to a rebuilder. Have your preferred gearbox rebuilder source the core for you. You can thank me now.
I recommend, I have NO financial relationship with them whatsoever, California Motorsports. Ask for Rodger and you can tell him I recommended them. Other forum members have recommended additional US based Porsche gearbox repair shops. I have no negative experience/comments with other shops.
https://californiamotorsports.net/
I definitely want to have something that I can get repaired easily if ever needed.