Rear end fitment

[donthekawguy]

I have a 66 short bed and the rear gears are way too steep. I can only cruise at 45 mph and the rpm's are 2900. The PO welded the spiders so it's also a full time posi. Instead of replacing all of the guts I can get a rear end out of the local pull it yourself yard for $100. I need to know if all of the 60 to 66 rears are the same or what rear end will bolt right in. I also have a set of 6 lug ralley wheels I want to keep for a couple years until I do a frame up resto on the truck. The yard has a 61, 62 and 63 truck in the yard so I'm hoping one of those will fit.