i was given an 8.5 10 bolt rear ,out of an 1970 chevelle.it has an 2.56 open carrier in it now.the question is. can i pull the 256 open carrier out and bolt in a 3.73 posi unit in the same housing? or are the housings dirrerent? and if so. what year/and model of carriers, will fit this housing?...thanks in advance for all your help ...sasquatch58

Yes you can, but the '70 Chevelle had the 8.2 rear end, not the 8.5. The 8.5 was used from '71 to '98 and all gears and differentials will interchange. The axle splines were increased from 28 to 30 in '90, so watch out for that.