Hi Tom, well my engine is tuned and so far with the stock 650 GB looks like it needs a longer ratio, even at steep hills and I mean steep little car like to accelerate than decent so my guess is that 9/39 will fit perfectly. I am not in a hurry though since my car now is at the shop for general reconditioning and painting so I will be busy when I get it back but I will think of it, maybe find a second GB rebuild it and swap it I'll see.