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

I agree the compression is probably lower than you think, unless the block was decked, a lot. These engines are almost always further in the hole than spec. Have you done a compression test?

Did they tell you why they thought it needed to be richer? Did they hook it to an analyzer, or did they just guess? You could put the original jets back in and you should be able to compensate for the accelerator pump mod by playing with the stroke. But if you care about mileage I'd get a Carter carb.

I picked up almost 2 mpg over the mopar electronic ignition by installing one of those cheap chinese HEI distributors from eBay.

Your ignition timing sounds right. Why anyone would plug the vacuum advance is a mystery to me.