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
One note on this info------ the early 60's 9" rears look much like the 8" rears, in that they don't have the "hump" in the rear of the housing that the later ones had, So if in doubt, use the socket/wrench method to determine for sure which rear you're trying to ID. You wouldn't want to pass up an early 60's 9" in mistaking it for an 8" I have one such early 9", that I found under a 66 Stang parts car. It's an early 9" from a 57-59 Ford and it had a "WAR" case center section that lacks the "N" found on the Nodular cases, making it look nearly identical to a later model 8" center section. So in trying to ID it in the mud in a pasture, with a bumper jack resting on a rotten board holding the car up so I could look under it, I almost passed on it. Good thing I didn't, it also had 3.70 gears and a Detroit Locker in it.