How Much Horsepower Required to Cruise Comfortably at 70 MPH?

The power required will not change. The power supplied would. I'd imagine you need probably about 100 crankshaft hp to run a B body steady at 70mph. So the 318 might need 1/2 throttle to do that. A 340 might require less throttle, because the engine will be making more power at the same throttle as the 318.