7&1/4 8&3/4 gasket interchange Q
rock has the stuff, or napa/ORLY's should as well.
fyi, rock is gonna bang you up on shipping so only go that route if you're getting other stuff.
but if you need other rear end stuff specific to the build or as spares dr. diff has the stuff as a complete kit at a super reasonable price.
felpro #'s 55032 (axle flange) 13900 foam backing plate/axle retainer