Changing gears isn't going to get you more torque. Going up in size will get you higher rpm at highway speeds and more get up and go, lowering gear size will slow you down off the line but get better gas milage as rpm will be lower when cruising.
Posi isn't going to change anything unless you have the power to spin tires easily, then it'll apply power to the other tire, doubling the amount of resistance to the road, no more spin.
Best thing for low end torque increase is keeping the stock manifolds but using a decent breathing 1½" dual exhaust, high flow cats and H pipe.
Make sure the carb is tuned well, and timing set well. Anything more than that will require motor work.