I'll second the Detroit Locker suggestion. I've had one for several years in my '69; it works as advertised and has been totally reliable. Be aware that with no clutches to soften things, you can sometimes feel and hear the locker working as you back up or go around corners. Its not really harsh or noisy, but something to be aware of. Likewise, the car will handle a bit differently, since the inside rear wheel no longer slows down when negotiating corners (the outside wheel has to speed up, to provide differential action.) Again, not really a problem, just something to get used to.
BTW, "positraction" and "posi" are GM words. The generic terms are "limited slip" or "locking differential." "Traction-lok" is Ford speak for the same thing.
Lastly, NASCAR teams sometimes sell their used Detroit Lockers (as well as 9" ring and pinion gears) on eBay at reasonable prices. They're usually 31-spline units though, and its also important to get one with the correct bearing size for your housing. But if you do your homework, you might find a bargain.