You are looking at 9-11 in town and 14-16 on the road. You can replace the points/condenser with an aftermarket electronic ignition module. Your plugs will last longer and you'll eliminate the ignition maintenance. I did it on a 74 Pont Gp with a 400 CID motor.and it was great. It also helps the mileage a little. Also try a K & N aftermarket air cleaner. It pays for itself because it never has to be replaced, and it reduces airflow restriction which will also help mileage a little. Slick 50 the motor, and pump up the tires a little. You should be able to increase mileage 2-3 mpg. Its not a lot, but hell, these days of $3.00 a gallon gas, every little bit helps.
72 electra, 455, turbo 400 trans, probably a 2.73:1 rearend, will go 18ish on the hiway and 10-13 in the city.
I had a 72 olds ninety-eight(basically same car) that got the above mileage, untill i put a cam in it and had the heads milled with a re-jetted carb, then it got about 5 mpg in the city(i couldnt keep my foot out of the gas!!!!!!!!)