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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: https://www.thirdgen.org/forums/tran...-10-bolts.html (FAQ About 10-Bolts)