Most likely a 2.x:1 (2.52/2.73). With the drums off if you rotate the driveshaft one complete resolution and count how many times the wheel goes around will tell you the ratio. With an open rear rend this is harder since both wheels need to turn the same amount to get an accurate measurement. Or, just pop off the differential cover and count the ring and pinon gears. Hope that helps.
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.
I've had the exhaust set up for a few years and recently fixed a couple vacuum lines. The timing is spot on as well as the carburetor. The only thing to do is save up for the 400 sb i reckon. Might go up for some highway fun bit most likely going to redo the interior next. Thanks man