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

Look, something that seems to be missing here is "making the most of what you have."

FIRST step in that process is to determine what you DO actually have.

1---Run a compression/ leak down test to get some idea of the condition of the engine.

3--Use a piston stop to verify ABSOLUTELY that the timing marks are correct

3--Determine if the cam is in time. You can get some idea of this by determining the cam specs, and by marking the balancer or even tearing it down enough (pulleys) to mount a degree wheel. Pull the driver side rockers to relieve pressure on the lifters, and use an indicator on a rigged--pushrod device to determine when the no1 valve events happen

4--Most important of all is to completely go through your ignition timing and determine "what you have". You want to find out what you have for initial timing, (at idle), what you have for mechanical advance, and how much the vacuum advance is applying. Along with this effort, inspect and make sure the ignition system is in good shape, wires, plugs, cap, etc

5--AFTER you have done all of the above, go through the carburetor. Read the plugs, experiment some with jetting. Jets are not all that expensive. Make certain that the float level is correct.

6--Go through and KNOW for sure what you have for gears and tires, and verify that the speedo/ odometer is correct. If any in your family has a GPS, you can check the speedo against the GPS.