318 drinking gas like hell

To many variable here to even guess..................and as Red said

gearing of the van...........

what sort of driving...........if this is not highway driving, then the figures mean nothing

what is the timing, the advance, that is the total advance, is it working correctly, and is the vacuum advance working?

What is the manifold? Torker, example, would be a poor choice for "torq" engine and mileage?

What are you running for camshaft?

Could it be that the timing drive is worn?

How many miles on the engine, what shape is the engine in? Bad valves? Worn otherwise?

And if you folks are using alcoholized gas, that does nothing for mileage, either

And..........................on........................and .......................on................................