How Much Horsepower Required to Cruise Comfortably at 70 MPH?

Jadaharabi said
Won't the torque converter affect the RPM when you change to the 340. That being that you stick with the same torque converter.

same converter will not be affected cruising at 70 mph. Only way the 340 (or any engine) will affect the converter will be the amount of torque of the engine will "flex" the converter under heavy acceleration.