[donut]

Start here, FAQ: SBC Ignition Timing. In case you haven't figured it out yet, a lot (relatively) can be found in the "tune".
Other things that seem minor, but do accumulate. Fresh fluids front to rear, tire pressures (you said stock, so I'm assuming no wide tires), brakes, exhaust...

Like I said, minor, but may squeak out another mile or so.

The '73 I used to own would knock down 13 mpg on a regular basis. Vortec heads, mild RV cam, headers, dual exhaust. Np205, turbo 350, Dana 44, 14 bolt and 4.10's.