RPO was G80. Not in any package options from GM for the G-bodies, and always had to be ordered separately. GNX was the only G-body it came standard.
If your gear set is still in good repair and not worn out, you can reuse your gear set. That would save you some $$ right there. I'd probably get new bearings and seals at MINIMUM if you're going through all that trouble, though. If you can find a used GM posi center carrier from somewhere and associated guts it'd be an easy swap. Casting number for the center section is, IIRC, 410408 for the posi carrier that can handle 3.73. It has the S spring and the flat, stacked clutch plates. (Don't get the 410409 - it's for 2.56 and numerically lower number gears. It would work, but you'd need a spacer. Not a great idea) The GM carrier is pretty much a drop in. Housing is the same for posi or non posi. Of course, you'll have to set it up properly once you do all this work. That's a whole nuther ball of wax if you're not savvy at doing a job like this.
For aftermarket stuff, I would advise buying a full installation kit from places like Ratech, Richmond, etc., so you can have all the new bearings and seals/gaskets and bolts and other junk. I'd probably buy a couple extra crush sleeves to have on hand while you're at it, just in case (GM p/n 1234726 or aftermarket equivalent). They're not all that expensive, around 3 bucks, and it's easy to over-crunch them when installing the pinion. And if you don't need them, you can always sell them. Auburn makes a $400 posi rear carrier that your factory gears will bolt to with 28 spline axles. Auburn p/n 542018. It should be good up to 450 HP easy. If you're going to go higher HP, I'd suggest getting an Eaton.
OR- you can always farm it out to someone who is good at doing rear end work.
I'm no expert. I'm just thinking out loud how I'd approach this if I didn't care about originality and having to use GM parts.