converting an 8 3/4 to the correct sure-grip

You members have been great on alerting me that the original Mopar sure grips are out there and not be talked into the thou plus local shops modern suggestion. I've just located an original 8 3/4 posi from a 489 case (the one which I have w/o posi) which the seller says is in excellent condition and been in storage over 15 years since he swapped it out for a spool? What does that mean? It's got the tapered bearings and is for 30 spline axles which I believe is what mine should be. Shouldn't I expect if I drop my original 8 3/4 and this posi diff off at this local "reputed" diff shop that they would pop right in (at a low cost?) and benefit with an overall inspection of the whole rearend and these "used parts" to feel "correct and safe" to pop it on my new spring kit. (And of course get the driveshaft modified). Thanks for all your help, Frank.