You don't need a new case to install either a spool, locker or sure grip. Anything made for the 8-3/4 will work in any 8-3/4 case.
Do you have a dial indicator and a good accurate torque wrench that goes to 100 lbs and a spanner wrench to fit the side adjusters? That and a good way to secure the differential while you work on it and a few hand tools are all you need.
First you need to clean the gearset real good to get any oil off that will interfere with readings then set up the dial indicator to check how much backlash the gears currently have. I do it in a few locations to verify it. Write down the B/L as this is the setting you'll be shooting for when you assemble it.
Then mark the caps as to what side their on so you don't get them mixed up and of course mark the case accordingly. I use number punches but even a center punch to put one dot on one side and two dots on the other is sufficient. It's imperative not to mix the caps up
Next remove the 5/16" bolts that hold the adjuster locks then remove the caps holding the unit in place.
Now remove the bolts that hold the caps in place and remove the carrier.
Remove the ring gear bolts, note they are left hand threads. Gently tap the ring gear off the carrier with a brass hammer.
Thoroughly clean the ring gear and if you can get a 3/8-24 left hand thread chaser it's a good idea to run the threads in the ring gear. Clean the caps and bolts to prepare them for reassembly.
Install the ring gear on your new carrier using a little red Loctite on the bolts and torque them to 55 lbs.
Assuming you already have the side bearings installed on the new carrier drop it in place and install the bearing caps with the bolts finger tight.
Lightly oil the adjusters and thread them in being careful not to cross thread them.
With the dial indicator set up to read backlash evenly tighten the adjusters until you have .001-.002 backlash making sure to align the adjuster opposite the ring gear so the lock will bolt up. Spin the differential a few turns to verify your reading and readjust if necessary. Now tighten just the adjuster on the ring gear side until you achieve the backlash reading you originally had. When you get close it'll be pretty hard to turn the adjuster but that's what you want to properly load the side bearings. Most gear sets that were operating good will have any where from .008" to .012" backlash. If the original backlash reading was more than .012-.013" you may find it necessary to loosen the adjuster opposite the ring gear one notch to achieve the backlash you need. Once the backlash is set and verified by spinning the differential a few times then checking it every 90 degrees you can torque down the cap bolts to 90 ft. lbs. and install the adjuster locks using a little dab of Loctite on the bolts and your done
There are quite a few video's on YouTube showing this done. Watching a video might help to see it done before you do it. If you can't find one on the 8-3/4 the Ford 9" uses the same style adjusters and locks
One last thing. I totally agree with Locomotion on not using a spool on the street. Their not made for street use. The first time you get caught out in the rain you'll learn real quick why their not made for the street
