Most any seat with the correct width will fit but I've heard that 80's Lincolns fit well. A fellow on the east coast used seats from an 84 or 85 Town Car and they look good. As for rims, KF used the same bolt pattern 15x5.5 as Chrysler so wheels from Chrysler products will work. You will need to remove the guide pin from the drums if your new rims are not set up to accept them.
Places like AutoZone and NAPA carry gaket sets for KF engines. Hard parts like pistons, etc, can be sourced thru anywhere that sells TRW parts. Other brands are available too and NAPA may have them as well.
Carb kits can be purchased from Rudy Phillips in CA. I don't have his contact info at hand, but I'm sure someone else will chime in.