With a 3.80 gear to run 70MPH you will be running right at 3,000 RPM, the same calculator sez 2500 RPM @ 70 MPH would be a 3.18 gear. You can pick a ratio from there. I've ran the Auburn posi units in some lighter cars with no problems, fairly sure they have one for an 8". As far as setting in up it's like any other rear end, if you don't have a pinion depth setting tool, probably should farm it out.