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

It sounds like you have two conversions going on here. The first is the 7 1/4 to 8 3/4 swap which has been discussed extensively on many threads lately. The other is to make an existing 8 3/4 a limited slip rear end. Assuming you have the gear ratio you want, it's fairly easy to add a sure grip. If all the bearings and other hard parts are in good shape, it's only a 2-3 hour job for anyone handy with tools and has a good manual. Two important areas are heating the ring gear to install on the new carrier and a dial indicator to set the axle shaft end play. Don't ignore either one! The last time I checked, you could get a new cone type carrier for around $350 or so. Used clutch types are available from time to time on E-bay, Craig's list, Hemmings, etc. The cone type works great when new but when it gets worn out there is not much you can do to it to make it like new again without spending almost as much as another new one. The clutch type can be rebuilt with a new clutch pack kit for around $50 the last time I checked. If you'd rather send it out to have converted, either Randy's or Moser know how to do it right. Don't use the green bearings for a street car. They don't handle side loads very well. $1000 sounds a bit excessive though....even with new bearings and seals. If you're going to put more than 500 horsepower through it, Moser has some nice upgrades to handle it.
Thanks, Mark