Best possible mileage?

Take a look at the camshaft. Compare the duration on the compression and powerstrokes. IIRC the cams are (318 first then 360);
240/130/122/248/20 ; compared to
252/123/119/256/30 ;That's ;
intake/compression/power/exhaust/overlap
>Notice the 130 compression on the 318 versus 123 on the 360. This makes about the same pressure in each engine so that's a nevermind.
> but notice the 122 power on the 318 versus a slightly smaller 119 on the 360. Again, the slightly longer stroke of the 360 makes this to be about the same actual physical distance.
> Now look at the Induction duration; 240 versus 252. That is is 12* or nearly two more cam sizes. That's a lotta extra time to be pulling air thru the primaries. And that air always has fuel in it for the same amount of time. So if there is just a tiny bit too much fuel in it, it gets 12* extra time to pull it in.
But worse is if the peak cylinder pressure does not occur in the 25 to 28 degree ATDC window, cuz the piston, with the longer stroke is rapidly accelerating away from TDC, and past the optimum window.

If your vehicle takes 50 hp to cruise at 65 mph, then it should make no difference what size engine you use, 50 hp is 50 hp.But it does make a difference; why? Because we are talking about 50 hp NET,off the back of the crank.
So if the 318 pistons and valve gear sucks 10 hp and the 360 sucks 15 (I mean I'm just throwing numbers at the wall to illustrate the point), that in an increase of 1/3 or 33% more internal friction. So the 360 HAS to use more gas just to spin itself.
Then consider that a 318 usually comes with a 904, versus a 727 with a 360, and the 318 will have a 7.25 versus an 8.25 with the 360, So again, the 360 has to drive thru those handicaps.

But if you make those considerations, as I have, when I installed a 318 top-end and cam on a 340, then you get the 318-type fuel-mileage out of it.
Or if you slow the piston speed down of the 360 to match the 318, again you get 318 type of economy.

But what of the 225 with it's 4.125 stroke? Yeah, there it is . Compared to the 3.315 of the 318, that is a lot of ringtravel
In a 318 the total ring travel over two revolutions of just one cylinder is 3.315 from top to bottom to top to bottom is 13.26 inches versus 16.5 for the slanty. When all eight are considered, that comes to 106 versus 99, so the 318 is only 7% more. It wouldn't take much frictional difference to put them on par, even considering the bore difference of 15%.
The point is this; Sometimes,more is less. Sometimes 318 cubes, over 225, uses less fuel per mile

The one single fuel waster is not having peak cylinder pressure occur in the window of 25 to 28 degrees AFTER TDC. Put yours there and see what happens.
I would like to tell you that I once did, and gearing my reasonably shaped 68 Barracuda to 85=2100 rpm(see edit1) with a manual trans, she returned to me 32 mpg. But nobody believes that so it must be impossible. And this with a 360 and a 223/230/110 cam yet....... so you didn't hear that from me.
My cruise timing? set to 2100 =45* in the distributor plus up to 15 more on the dash-mounted dial-back, spark-delay box, so 54* on this trip. That's what it took to put the peak pressure inside the window.

Edit one . I geared the car to go 85 mph at 2100 rpm. That was with an A833 overdrive box, ( .71od) and a GearVenders Overdrive (.78) behind that, and 3.55s out the back, so a final drive of 3.55 x.71 x.78=1.966 in double-overdrive or Second overdrive; same thing. This would be like running 2.84s with an A500 in loc-up.