The G50 is stronger but the 915 would be better if your going to use a porsche motor. The 915 will handle anything it can put out short of twin turbos.
The factory torque specs (VERY conservative) are
915---- 237ft.lbs.
G50---- 300ft.lbs.
typically the 915 is good to about 350hp and 350ft.lbs torque, while the G50 goes to almost 500.
The 915 can flip the ring gear for reverse operation. The G50 cannot and has to be run upside down.
The 915 weighs about 100lbs., the G50 about 125lbs. The 915 is lighter and smaller and costs about 30 to 40 percent less.
Both are available with factory LSD, which usually adds quite a bit to the price.
'89 and earlier are the best years for the G50 if your going to run a V8. No bolt on gear reduction starter is available for '90 and later, and the stock starter won't turn a built V8. You can modify the later G50's to accept the pre '89 A.M. gear reduction starter, unless you do it yourself it's about a 500.00 machining operation.
Another thing to look out for with the later G50's is some come with an extended mount out the back which won't fit within the GT40 body. This can be removed but again its another machining operation.
[ November 05, 2002: Message edited by: Kalun D ]