340+ for the STREET

I did it in an E body... Also did it with a 440+6/auto in an E body... It's called driving. I prefer sticks in snow and ice, even in fwd cars.

Edit - Also in regard to the mileage - my statements include a mix of city and highway as reported by the owners.
Some examples:
A 3900lbs Ebody with a 496/727/3.23s gets 14 mixed driving as reported by the owner.
My 340/4sp/3.23 Ebody got 18 mixed driving, ran low 13s, got 23+ highway, and passed emissions.
My 440+6/727/3.91 Ebody got 16 mixed, ran low 12s, didn't ever measure highway. Drove it year round and it had snow tires for winter.
A different 3800lbs E body with a 422" small block/727/3.23s, runs 108+ in the 1/4, factory type heads, "non-modern" hydraulic cam, gets 17 mixed, but biased towards highway.
Obviously highway is most economical but if you can't get 15mpg out of a street-type small block build of any size in an A body you need to learn to tune and make sure the rest of the car is "right". Biggest killers - not running a vacuum advance, running with under-inflated tires and worn front ends, and poor timing and carb setup.