Your drum brakes will swap over IF the 8 3/4 has the same axle stud bolt pattern. You will need new brake lines if the bigger housing you have does not have them.
I wondered about that. Just to be sure you are understanding I have 7 1/4 rear now with 5 x 4" bolt pattern (not going to change) (67 Dart Convert GT 273 2bbl rally package, OE 4 piston Kelsey - hayes front disk)
