Fuel Injection - explain this to me like I am 6 years old

That 1971-318 was rated at about 9.2 or 9.5 compression ratio IIRC. The idle vacuum should be greater than 18 at 700rpm. It should idle down to 600 in gear, and I'm guessing, still better than 16". And these numbers are for the factory timing specs, which I think was TDC. The factory cam was a 240/248/112 with an IVC of 50*. If I got all that right, then

Static compression ratio of 9.2:1.
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 8.07:1 .
Your dynamic cranking pressure is 161.70 PSI
V/P (Volume to Pressure Index) is 133...............................................133

At 162psi, that 318 should run gangbusters, peaking at about 22inches or better with just a bit of rpm.
Assuming a decent LD, and a free-flowing exhaust;
Throw a bit of timing at it, then start leaning it out. Before you know it, you'll be at 16/17 mpg. Then bring the minimum running temp up to 195,and feed her some fresh cold air, and pick up another 1 or 2mpgs. Then try and get the cruise timing up to about 54* of timing at 2250 , and then your point to point should be approaching 20mpg. Then start leaning out on the cruise step until it won't hold a steady speed, then give her a bit back and Voila, I see 22mpg on the horizon.

But I gotta warn you;
If your compression is down, Or your LD is bad, or your exhaust is plugged, or your Dist is sick,or your tires are flat. or your brakes are dragging, or your tranny is slipping,Or your dragging the tires sideways down the hiway,or your running N50-15s at 20psi sticking 2 inches past the body-line,, well then maybe 12.5 is all she's got in her.