look for the "hump" offset from the center on the rear of the housing on most housings you will see on cars from the 60's & 70's - make sure you measure what width you will need, whether the housing has a filler hole or the hogshead has a filler hole, axle size 31 or 28 spline, posi or not (jack it up and spin a wheel while both are up and if they both go the same way it is a posi and if not it is a standard rear) etc... most trucks have 9" rears but lots have 31 spline axles - make sure what you want before you get heavy into it - there is nothing wrong with a 8" for street use - you can get a posi unit for these with good gears from different sources - also you can get a "locker" from powertrax - much better than original ones and good gears 3.55 etc can be had from a mustang II or 9" gears best found in Bronco or F-series p/ups..... good luck