Welcome to TGO.
You can't swap the disc to a drum rear without modification or Ed Miller's kit. Remember too that you'll need a disc proportioning valve, e-brake cables, and driven and/or driven gears to make the speedo rear correctly.
BTW, the flanges/backing plates are different between 9- and 10-bolts. Nine-bolt are a bit stronger, but parts are apparently difficult to come by. The following FAQ will provide more info on 10-bolts: http://www.thirdgen.org/techboard/tr...-10-bolts.html