The cop spec 318 in an 87 Diplomat was a lower compression ratio than the 2bbl version, I believe it was "blueprinted" at 8.1:1 vice the 9.0:1, which in real life usually means about .2-.3 lower than blueprint.

Point is, your goal should be very attainable. City mileage should be much better with the lower first gear of an OD A833. You probably already have hte Diplomat beat in weight, too. Funny thing is I sold that Diplomat and it's 318 is going into my 65 Cuda, one day.