In a car as light as a T bucket, you shouldn't run a posi rearend. A normal, non posi rearend is designed to let one wheel turn faster or slower than the wheel on the opposite side, because when you go around a corner, the wheel on the outside of the turn actually has to travel farther than the wheel on the inside of the turn. This means that the wheel on one side of the car must be allowed to turn faster or slower than the wheel on the other side. This same built in mechanical ability will allow one wheel to spin more than the other when under extreme acceleration.
The positraction rearend was an attempt by Detroit to limit the amount of unequal spinning during hard acceleration---to do this they put friction clutches on the "slippage" mechanism to limit the amount that one rear wheel could turn without the other one.
This worked fine under hard, straight acceleration, and in a heavier car, the clutches would still "slip" enough to turn one wheel faster or slower than the other while going around a corner, as required.
On something as light as a T-bucket, there isn't enough load on the rear tires to activate the clutch mechanisms when turning a corner, which means that the rearend keeps both inner and outer wheel spinning at the same speed.
This makes the rear of the car skip and slide when going around a corner, as the rear tires now have to slide on the pavement to allow one to turn at a different speed than the other. THIS GETS REAL SQUIRELLY, REAL FAST!!!!
If you have gobs of torque being transmitted to the rear wheels on a light car like a T-budket, you will still have to be really carefully on the loud pedal anyways, but a stock 350 engine running through a powerglide isn't going to be that scary anyways. I recommend a gear ratio of 3.55:1, definitly not a 4.11:1 for what you are doing.