I recently purchased a rear out of an 84 trans am . It is a 3.73 open diff unit. I would like to put a posi unit in before I install it. I beleive an 84 rear is still 7.5? with 26 spline axels. What do you measure to determine 7.5 or 7.65? Assuming it is 7.5 what posi will fit? Are the two size carriers interchangable?

7.5" and 7.65" are the same thing. Same carriers, same everything. Only difference is, at some point in the production life of that model of rear, they decided to make the OD of the ring gear a tiny bit bigger.

Any posi for that model of rear will fit. Howeve, it just has to have 26 spline side gears, to match the axles. Or, you have to change the axles. Which, if you have to buy axles anyway (like if yours are already bad as they so often are), it makes sense to upgrade to 28 spline instead of directly replacing them with the weaker early-model stuff. Like these.