You're going to get a million diff. responses on which shop to go to. Just do some research, ask around, and call each one and deal with who you feel comfortable with. I've personally had very good experiences with Need 4 Speed and Coastal.

As for gears, I'd def. say 373 as well especially with a power adder and what else you want to do....and the fact that you already have traction probs on street tires.