1966 273 v8 cam suggestions and exhaust too?...it will never be super performance but
Rumble is right about the exhaust.......listen to some cars and get a idea what's going on with some of the newer mufflers.
As far as the cam, you want to stay pretty mild on a street style low compression 273, that has a stockish trans and gears. I assume yours is a hydraulic engine, so off the top of my head, i'd say the mild Voodoo should work out pretty decent.