built up 318 oddly low MPG's? hp rating?

Mine:

67 Dart, presently "Craigslist" 318, all stock 70's engine except---

mild solid cam, similar to the 68 340 cam
Edelbrock Performer
was Ed 650 cfm

3.89 (Ford) gear, 225 x 75 x 15 tires, so 28" tire, runs about 3000 at 65 mph.

Dist is Accel / Mopar look-alike, with 20 or so mechanical. 35* total mechanical plus vacuum.

I don't worry much about mpg, and most of my driving is "short hops" around the area. A few days with mostly highway, it gets over 15, bumping 17.

There is something terribly wrong with your combo, and as others have said, most probably very low compression, but also the cam. I don't have any idea how that manifold "is" for that combo.