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

OK whoever disconnected your vacuum advance for more power was such an idiot that nothing else they did should be viewed as anything other than wrong.

I can't imagine a mild 318 needing larger jets or a drilled out accelerator pump. I would suggest returning the carb back to the out-of-box settings and starting over.

The engine likes 14-35 ignition advance because of the fairly low compression ratio. (X or J heads with only .010 cut off and composition head gaskets will end you up with more like 7.8 to 8.0 compression)

The large ports and intake runners WILL hurt throttle response, but shouldn't have a major effect on fuel economy unless some brain donor set the carb up ridiculously rich in an attempt to compensate. I wonder if that isn't what happened.

A 318 with an XE262 cam and 3.55 gears should get closer to 20 mpg.

How long did you drive it with the kickdown linkage set up wrong? Nothing kills mileage like a screwed up automatic transmission.