How Much Horsepower Required to Cruise Comfortably at 70 MPH?

I have no fancy formulas at all, only a full year of real life experience with a 70 340 Challenger. In 1981, I was dating a girl who was going to Austin Peay State University in Clarksville, TN. That was an over 300 mile trip I was making every other weekend. I took the 3.23 Sure Grip out of the Challenger, and installed a 2.76 single legger. The 340 was factory stock, from the air cleaner to the pan with factory HP exhaust manifolds. I could cruise at 70 on the interstate at 2100- 2200 rpm. The car would get 20 mpg at that speed and rpm. That's all I got....lol. Still have the Challenger, and it has the 2.76's in it to this day! Haven't driven it in years, as it's in a million pieces awaiting resto. The girl? She became my wife, and is now the :realcrazy: ex wife....lol.