i have a stock 91 RS 305TBI
and it doesnt have a posi rear end.. my questions are ifi bought a Posi unit. and some new 3.73 gears. would i have to change anything else or would the posi unit make it so both wheels spin from the begginin?? as you can see im not good with the car stuff.

 

that's what a posi does, make both tires spin

 

What gears do you have now? If you have 3.08 or less you will need a 3 series carrier for your diff to put in 3.73 gears.