That is pretty bad mileage, but a closed choke is a good reason why. My 460 in my 70 is getting 18.6 @ 70 mph with an AOD and a 3.50 gear. It still runs a little rich on the cruise side. Gets about 12-13 around town. It can still be better.
I think with the 302, C-4 and 3.00, you should get about 17+ on the highway. Aroud town you should get abou 11-13. To accurately test highway mileage, top-off your tank, cruise a known distance, then top it off again.
Your ignition timing can make or break your mpg's. Make sure it is adjusted properly and if you want better part throttle economy, then make sure to run your vacuum advance on ported vacuum on the carb. I like to set the initial timing higher then stock--about as much initial as it will start with at operating temperature. Then to have the mechanical advance swing out to about 33-36 by no later then 3000 rpms--test that with the vacuum advance unplugged. With total mechanical timing and vacuum advance should be about 40-44 degrees all in no later then 3000 rpms. Make sure to run a thermostat----no less then 180. To really set up the ignition your self, the best tool to have is a dial-back timing light.
Good luck,