Chevy first came out wiht the Posi-Trac for trucks in '59. Those optioned that way used a Spicer 45 instead of the standard Chevy rear. That was a one year only option, an Eaton sourced posi came out in '60, used to '62/possibly some early '63s. The 12 blt came out in '63, and was offerd with posi up to the last years offered in the late '70s ('77, I think). You'll most likely have a difficult time finding an original, they were somewhat rare. About the only way to ID for sure, because the "Use Limited Slip Oil Only" tag is usually missing, is to take the cover off and look. Best bet, especially if you're wanting to change the bolt pattern anyway, would be to find a '73-'87 1/2 ton to get both the front end and the rear axle from, then use an aftermarket limited slip unit.
