$700.00 isn't too bad of a price, but for about that amount you can buy a rebuilt center section ready to go. Then if your current center section is in good shape, you can sell it, trade it or hang on to it in case you have a need for it in the future. I'm looking to do the same thing right now and I have rebuilt a ford 8" rear end (which is basically the same as a 9" only smaller) and it is time consuming, but with a good guide and not being in a hurry it is easily done right. Parts will run 400+ dollars with a good grade ring and pinion gear, lube all bearings, shims, seals and track lok clutches.
If you look at center sections on ebay, be careful, many advertise posi but are in fact fitted with a full spool which you will find very unpleasant on the street.