Rear end interchanges?

[VA72C10]

I am looking for a posi rear for my 70. I know that 67-69 had narrower rears than the 70-72. And I've read that 73-87? Had 12 bolts that would also work (just have to weld the trailing arm bracket on) My question is in regards to 60-66 trucks....do those rear ends interchange onto a 67-72? Are they the same as 67-69? I am looking at a 66 project truck with a freshly rebuilt 411 posi...thinking of switching that rear for mine and then putting mine in the 66....The cost of the truck is what many are charging for just the posi rear.

I haven't gotten to see the truck and may never but want to know what rears will fit so when I'm scrounging craigslist I'll know what to consider. I did check the FAQ's and tried a few searches but didn't find this info. Thanks in advance.