It depends on what your looking for, you can get 3.73 gears but your gas mileage will suck you can go with 3.08 but the performance will suffer a little. If your looking for a good all around gears set I would go with 3.23. You can get a set for around $175 but I would have someone who knows what they are doing set it up.(no offence but I wouldnt even attempt to set one up) As for a posi unit, I would find an original GM one one ebay.
If you plan on rebuilding a engine with all new parts, you can pick up a 350 pretty cheap. Couple hundred bucks. 3.73's gears would be nice for performance. But like 73nova73 said, it will affect your gas milage. Id go with what 73nova said, 3.23 gears. Or 3.55's would be nice. They shouldnt be any more then around $200. My car has a posi lock in it, but it came with the car when I got it. So I dont know how much those cost. Not much tho.
1954 Bel Air 4dr 1954 210 4dr 1954 Chevy Bel Air 4dr 235I6/Powerglide 1958 Chevrolet LWB pickup. 235I6/4speed granny 1976 Chevrolet Nova 350/400 4.88's 1976 Chevrolet SWB 4x4 1/2 ton. 350/4speed 33's 1979 Chevrolet SWB 4x4 350/4speed 3/4 ton axles/Tcase lifted, 36's 1981 Mazda RX-7 1996 Ford Ranger SWB Reg. Cab 2.3L/5speed