You'll need a posi. No matter what type of motorsport you're into you need power to both wheel. In cornering situations, you'll unload the suspension on the inside of the turn, lifting the inside rear tire creating the dreaded "one tire fire". Even my '73 Vega that I autocrossed for many, many years could lift the inside rear tire off the ground and without a posi, it sucked. It had a posi when I bought it new, but I have the stock 6" rear in the car, and when it broke, I didn't replace it. I do have a new one for the car to be installed if I ever find the time.