I would consider a 3.00 gear if you are doing a lot of cuising. 3.50 is a little low for my heavier Mustang with 289 and a T5 5 speed. Second gear comes up too quick while cruising. With more torque and a lighter car, I think you would be happier, and really feel the acceleration between gears instead of shifting every 1-2 seconds.
I had bought a 4.56 traction loc unit at a swap meet with the intention of changing the gears, but I wanted to try it out before investing the money in it. I put Royal Purple
synthetic gear
oil in it, and after a couple hundred miles it looked like a chocolate milkeshake. I was told not to use
synthetic oil, especially with a new gear set. I do not use any friction modifier in this set up, and have no chattering, but I have experienced the problem in an 8.8 rear end.
Tha hard part for me, buying at a swap meet was identifing the posi units. I would research on the web to find pictures so you know what you are looking at before you go. I saw 2 that day, one with a 3.50 gear (sold out from under me) and the 4.56 that I bought. both were in great shape, and sold for $300. another $300 to have new a new gear set put in, and I saved $300 over the best price I found on e-bay, and a reputable shop set it up for me.