Mpg on a mild 360?

It will depend on the tuning, the throttle, and the convertor. My impression is it won't be more than 10mpg if it's an older type "10 inch" 3K stall. Witha tighter unit (so cruise rpm is above the flash stall rating) then you will do much better assuming the carb and timing curves are spot on. I'd also be running the vacuum advance - without it you're giving up anywhere from 10-30% in mpg.