Hello Ramsay,
Thanks for your message and inquiry. In terms of the best final drive, I would advise getting the 4.9 as it offers the most torque multiplication for the rather torque-less B16B. Be sure to clean the factory magnet inside the transmission case and I would also advise that you check the input shaft bearing and the differential/LSD carrier bearings. We also sell the LSD bearings, and a set of our powerful transmission/sump magnet plugs are also a good idea if you don't already have them.
As for who to purchase from, it's entirely up to who you are most comfortable with.
Regards,
Patrick