Super Six Fail

Are you going through the correct procedures to see if you have a vacuum leak? Buying a carb kit isn't going to do much of anything if you have vacuum leaks. And just because the throttle shaft is tight, doesn't mean its not leaking air. Troubleshoot the problem one step at a time, and start by doing the checks that don't cost you anything.

With the motor running, spray some carb cleaner on the intake runners, base of the carb and the throttle shaft ends and see if the motor increases rpms when you do this. It's pretty much free to check and should be done before spending any money on the car.

PS, the exhaust can net you 1-2mpg in the end, but don't even mess with that until you get this thing at 20mpg and running smoothly.