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

A double pumper is pretty much the opposite of fuel economy. These guys http://www.holley.com would never have recommended that carb for your combo.

You'd need to take a pretty heavy cut on those heads to bring compression up. Like .060 or so. That can get expensive when you add the costs off cutting the deck surfaces, intake surfaces, shorter pushrods, gasket set, etc... Then there's the issue of the big ports. If you had a 3000+ RPM converter, 3.91 or deeper gears and a cam with more than 230 @ .050 duration you could take advantage of the flow potential. But you don't. A mild street 318 generally does better with 318 heads. Well prepared 318 heads can support 300-325 HP pretty easy. 360 heads will support 400-450, but will hurt low end response on a 318 (if you're building a 400+ HP 318 low end response won't exist so who cares).

So what should you do? Cutting the heads you have now will be expensive. Swapping 318 heads, a regular Performer intake and a more reasonable carb could also be expensive. You could always leave the heads alone and swap in a 360 short block...

3000 RPM at 60 MPH seems a bit high for 3.55 gears. Are you running lowrider wheels and tires? A 26" tire should be more like 2800.