sb difference fuel consumption

hello, I have a 318 and 340 block, and like build the engine for daily use and long trip in highway (holidays). I think that 318 are more economic, but if little difference I want do a step away (340, 349 stroker or 390 stroker).
what is the difference MPG between a 318, 340 or some stroker with the same stuff... my car it's a early a-body with manual trans.

thanks...