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

Agreed on keeping the vacuum advanced hooked up. The 340/360 heads are hurting mileage two ways. They lowered you comp ratio and the ports/valves are to large. The velocity is slow through the intake and head to break up the fuel. (Atomize)

Your speedometer gear change may not be a 100% perfect fix. Slight adjustments can be made through tire size diameter changes. Also note that the older cars mechanical speedometers are never dead on perfect.

When the time comes, try a taller tire. There are speedometer cal ' or tire size calc's on line that will show the differences in change tire diameters. If your running a 215/60/15 tire now, a 235/60/15 may work out very well. Check the calc's on line.

A slightly taller tire will help in mileage. Very slightly though.

Add a MSD ignition to enhance fuel mileage. A regular 6A box will do well.