Super Six Exhaust

IIRC, the Super 6 exhaust system was 2¼" end-to-end. Classic industries has a 2¼" exhaust pipe listed. You can use a 2¼" tail pipe for the driver's side 340 set up. A 2¼" reverse flow (turbo) muffler will be quiet without too big an impact on performance.

Has the radiator cap and coolant recovery system been checked out? If not, the system could be puking coolant out and sucking air back in.