2.25" with a 2" crossover would be the ticket. Run some good flowing mufflers, not some unknown mufflers with unknown internal design. The 2.25" pipes will give you good flow velocity.

If you want to improve the exhaust manifolds, find some mid-90's manifolds from a 318 or a 360 - NICE design! I put these on my '73 Charger 340 Rallye - all you have to do is grind the center webbing on the manifolds so there is no obstruction coming out of the heads. I had to have a custom trans dipstick tube made for the RH side - there's a guy here locally that makes these.