Using 8 3/4 with /6

Nice looking Duster and welcome to the forum! You need the complete assembly from drum to drum and the shock plates (although some have elongated the holes in the 7 1/4 plates). I would install new U Bolts. For safety's sake, go through the brakes and think about installing new hard lines/hose available from several different vendors. I will be running a slant in my 65 wagon with 3.23 gears in my 8.75; others will probably chime in with some additional info.