Recommend a good Positrac unit?

[jnf2k2]

Title about sums it up. Looking to add positraction to my 76 K10 and looking for any recommendations. It has the stock 12 bolt rear end for a half ton and the unit would need to handle the 37" tires I have on it now. Any insight would be much appreciated.

Also would there be any other vehicle or truck produced that would most likely have positraction that would be compatible with my K10. I'm lucky enough to have a junk yard in the family so knowing where to look could save some money.