The 318 has never been known for good mileage. All else being the same what is the real difference between the mileage potential on the 318 and the 360?
I can show you B body bone stock 318's back in the day that got 18-20 routinely
I swapped a 71 high compression low miles 340 into my 70RR--which had been a 440-6, so heavy 4 speed and HP robbing Dana 60 with 3.54 gears and it got over 17 WITHOUT being careful