It takes a lot of torque to push a brick at 70 mph, your power band needs to peak at the rpm you are planning to run in while pushing said brick at 70 mph. Don't look at the horse power, look at where to torque is. I would want the motor be in 1800 to 2000 rpm range at highway speeds, so 3.73 - 3.90 - 4.11 would be about right. Carburetors also don't like low rpm in heavy trucks at 70 mph, the air velocity is pretty low so pulling the required fuel is pretty tuff. If you had a 454 or 500 inch Cadillac, then you could run around all day at low rpm due to the torque.
