Not sure

Build what you want, not what they badger you into.

Oh, by the way, a 360 can be had very inexpensively as well.

An easy 318 build up that requires no extra maching while still getting good mileage is easy. Swaping the engine in is the harder part.

My 318 Duster Project here; http://www.forabodiesonly.com/mopar/showthread.php?t=10508 Did get good mileage. but I never really had the chance to maximize the set up. Just a bolt in and go fun toy.

Basics to the build are;

1979 318.(With untold mileage)
600 cfm Holley & a 625 Carter were used at one point.
Edelbrock LD4B intake manifold.
Crane Cams 216/228-.454/.480/112 and there springs. Bolt on and in, no machine work needed.
Summit double roller timing chain.
MP distributor triggered by an orange box
Hooker headers into a Jegs 2-1/2 exhaust system.
I went with an elextric fan. It wasn't needed. But a V8 rad would be a smart thing to do. The /6 rad was new and had trouble in the 90* plus days.

No tranny mod but a deep pan.

225/70/14 sized tires. Gear ratio was a 3.21 in a 8-1/4 sure grip equiped. Rear springs by ESPO.com 1 inch over bend.

Untuned, it ran a 15.14 @ 89 mph. Got 18 mpg's running rich on a preety cool Nov. day @ Atco raceway southern N.J.

The stock low compresion engine was half the problem in terms of power making. Some good piston to head squezzee would have been nice for power and mileage. But an inexpensive buiuld is where it was at.

I beileve 18 to 19 MPG's was possible and I know 14's were in it.

Theres power and mileage in a tuned carb, degreed cam, a betterly tune distributor and a better ignition, in which I'd recomend a MSD or equal. This set up also could have use a wider and slightly taller tire to help against wheel spin. Which I had a bit at the line and a tad more for a longer leg. Which translates into a low RPM at the finishline, which I was maxed out and then a few hundred RPM by then.