This seems familiar I think you posted this question on FB as well? Glad to see you asking here you'll get much better answers.
Since you're running a 727 the mileage will never be that great and the difference in mileage between a 318 and 360 using that trans will be less than if you were using say a 904 or some type of manual trans. Also since you already plan to use tall 2.94 gears my vote is for the 360 Magnum. Just a prediction but if you built both a 318 and 360 to a similar level and put them in that car with that transmission and gearing, the 318 would get only slightly better mileage (1-3 MPG) while being noticeably less "punchy".
If you were willing to swap to a 904 then I'd lean more towards the 318 because you'll actually be able to achieve worthwhile gas mileage. Note 'back in the day' the slant-6 and 318 cars had lighter and smaller drivetrain parts (904, 7 1/4" rear end) because with a smaller engine frictional losses become more apparent and make the engine have to work harder than it should. Part of the reason why a stock 5.9L Magnum in a 4200-lb Dakota with a 46RE (727-based 4-speed OD auto) averages 13 MPG but put that same engine in an A-body with a manual trans and mileage "magically" goes way up to the 15-20+ range depending on gearing. Same goes for the G3 Hemi, even in stock applications a newer Ram 1500 will get much worse mileage than a 300/Charger/Challenger with the same 5.7L Hemi due to the weight, aerodynamics and increased friction from the heavier drivetrain parts in a full-size pickup.