Alright, I say we make this sticky.
Everyone that knows anything about gears on v6's post it, such as: what size rear end to get them for, what gears for 5pd vs. what gears far auto, what all you need...
The reason im posting this is because I am planning on getting them.
I would say probably 3.73's for 5 speeds and 4.10's for autos. Just because autos are 4 speeds, thus having a longer gear in the tranny to compensate for the 4.10's making them shorter. But what do I know, I'm just the import guy.
What he just said is good advice. You can also go with 4.10s on a 5spd but your rpm might be higher than you like if your car is a daily driver. Personally, I would use 4.10s in a 5-spd and definately use them with an automatic.
You need to get a ring and pinion set for a 7.5" rear end and a limited-slip differential such as a FRPP T-Lok. You may also need bearings, shims, differential lube and friction modifier. Ask your installer if these are needed and whether they supply them.