In the early 80s I bought a station wagon from my friend’s mom to grab the 390 to put in my 1950 F-1 Ford. It had over 200k on the motor, and the tranny was out; so I started to tear it down. The motor looked like it had just come out of the factory. Ended up just throwing in some seals, and it ran like a scalded dog until I sold it.
I did thrown on a high-rise manifold and a 750 double-pump. Top speed was only 90, but she hit that in around 8 seconds. You could actually watch the gas tank drain when punching it—six miles to the gallon was no big deal when gas was cheap!
My 390 is really fuelish, too. You’re right, gas mileage wasn’t a concern until the 70s.