71 demon 340 drum to disc

If you want to keep the 4" small bolt pattern to match your rear axle and/or wheels, get a complete setup off a '67-72 A-body. If you want the 4 1/2" big bolt pattern, a '73-6 A-body or '73-4 E-body is a good 1 piston Mopar type brake setup. You'll also need '73-6 A-body upper control arms for that conversion. There are also tons of aftermarket kits. some use the existing spindles with off-the-shelf parts and some are totally one-off, proprietary kits.