Since you have the 6MT you already have a 3.5 diff. The 3.7 will make a little improvement but honestly not a dramatic one.

 

Man, please don't mod your car, there are very few of these left unmolested. You really aren't going to do anything that hasn't been already done and that included dropping in a turbo/intercooler setup. Nearly every 350/G35 I see of our vintage has been modded and beat to death. If you want to be different, keep it as stock as possible and you'll retain a lot of the car's value. Save your money and keep it for maintenance items type of mods such as poly bushings, brake pads, stronger compression rods etc...

 

An Uprev tune will make it much more responsive and fun to drive. You can either dyno or etune it, for a daily car it really doesn't make that much difference

 

Lol wana buy her? I've already come to terms she's not a race car and I've lost the interest to try. She's clean and stock since day 1