Gear and converter would be the best bang for your buck in getting performance up. Beyond that I'd be looking at doing headers, cam, intake and then just driving it. A low CR 340 in a heavy 73 RR isn't going to be a ground pounder. Probably your best bet would be to keep the car stock and just use it for tooling around town or cruising on the highway. Sure you can do gears, converter, cam, intake, headers but you're still not going to have that potent of a package. Build your Dart for performance instead.