Engine size recommendation

I made an engine for my brother 3 years ago
he do the same with is than wanna do the engine
is pretty strong and tried to make cost as low as we could

1978 360 block
kb piston final cr. 10.57
915 heads ported at home
750 carb
.509 mp cam(not the best but ¸was cheap
air gap intake
1 5/8 headers you know the real cheap

he is running this engine for the last 3 years have alote of fun
good gas mileage except that he is running premium gas but
a little less compression it could run on 87 octane



what do you consider "good" gas milage ?? sounds too built for good mileage. my motor wasn't too far off from that and i was lucky to get 10 mpg.