I found a 26 spline gm posi unit out of an 82 cutlass. If I keep my 2.73 gears what knid of work will be have to be done to swap out my open end diff. Or should i change the gears at the same time and how much can I change them. I do not have any special tools at the moment so the easiest swap would be prefered.

 

Assuming yours is also a 26 spline rear, both are 7.5" rears,
the cutlass has a 2xx series posi also. It's fairly simple but requires a dial indicater and megnetic stand. You need to measure your ring gear "backlash" before dissassembly
and re-establish that spec on the replacement posi.
Note: (the pinion does not need to be touched.) Once you have that spec you can remove your carrier/ring gear noting the
position and order of the side shims. Transfer your ring gear onto the other posi. (An impact gun is easiest).
Install the posi in your diff housing with the shims in the
same position and order. Should be a snug fit. Set up your
dial indiator and check the backlash spec.
If it's different now you'll have to adjust it. If bigger shim the ring gear toward the pinion Right to left. If smaller , shim it away.
Note the order of the shims, try swapping one shim from each side.to the other (slightly different widths) and re-measure the backlash. If ya can't re-establish the backlash spec you'll need more shims to select from. Get them form another rear.
I think 7.5's and 8.5 use the same shims. Or get them from GM.
Either way the shims should fit in snug so there is no side to side play. If the posi is a clutch type be sure to add positraction type rear end oil. If its cone type it doesn't matter. Hope this helps.