How Much Horsepower Required to Cruise Comfortably at 70 MPH?

I had a 1970 Porsche with a fuel injected 1.7 liter VW motor (factory). The 5 speed had two overdrives (4th and 5th). I could not use 5th gear on the highway unless I was cruising at least 70 MPH because at lower rpm's it lacked the necessary horsepower/torque.

So I am thinking too far ahead with the 340 switch evidently (apples to oranges?).