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

You can pull the heads without pulling the engine, but have to pull and teardown the engine to get the block decked.

Regarding run of the mill (i.e. stock) pistons, there are two different specifications. The 71 and later LA318 pistons sit lower in the block than the earlier pistons do. I'd estimate a gain of 0.4 in CR by swapping those out.

The gearing thing is more than just the rear end gear ratio. It also involves the height of the entire wheel tire package. e.g. If you were to go to a larger tire, it would lower (numerically) the revolutions per mile of the engine. Compare going from a 205/60-14 to a 205/60-15. Rev/Mile 851 vs 817. 205/60-15 vs 235/60-15, 817 vs 772. A low number means fewer rpm to attain a particular speed. This is good for fuel economy, bad for acceleration.

Until you lock in on your wheel/tire package, getting the speedometer to read accurately is going to be like nailing jello on a wall. Once it is set, the rule of thumb is one tooth per 3 mph error. It the speedometer is reading 63, when your GPS says you're doing 60, need to get a speedometer gear with one tooth more than the one you have in the car now.

IMO, the carb is OK in terms of cfm. An air valve secondary is a better choice for a 318/auto combination. Consider having the secondaries adjusted so that they open later in the pedal travel.

One of the things I'm having to come to terms with in terms of setting up a carburetor is how to deal with ethanol laced fuels. It doesn't produce the bang for the buck that the old stuff did. There's a point where all I'm doing is pouring more fuel down the carb and getting nothing more from it. My goal is to stay just short of that point.