[84LX89GT]

I would only buy the Ford Racing gears because i've never heard anything bad about them. To install gears you need to have a dial indicator for checking backlash as well as a depth tube and gauge block set for checking pinion depth. You adjust the pinion depth first with the shims supplied with the gears, then adjust the backlash by changing the carrier housing shims. Also make sure if you do it, to do a pattern check with gear marking compound to verify you've done it right. If the gears are installed incorrectly they can break or make noise.
You could also have your gears installed by a shop for a moderate price.
I'd probably recommend 3.73 for manual and 4.10 for automatic. If you get gears for an auto, i'd also recommend your next mod be a higher stall torque converter, which would really wake up your car.