How Much Horsepower Required to Cruise Comfortably at 70 MPH?

I now cruise at 70 mph at 2500 with my stock 318 (2.71 rear end). I am planning a future 340. i don't know if the 340 will like cruising at 2500 rpm. I wonder if I should cruise in second gear (Torquefflite 1.45) with the 340, which would give me an effective rear end ratio of around 3.90 instead of the current 2.71?

In second gear I would be cruising at a lower 60 mph at 3000 rpm. To cruise at 70 mpg (in second) I would need a little over 3500 rpm, which might get a little noisy. (On the other hand, I see 3500 rpm with the 2.71 (in drive) works out to 100 mph!) According to the Camquest program dyno, 340 torque peak should be around 3500 rpm.

I wonder how much horsepower is required at 2500 rpm to comfortably cruise at 70 mph with the 2.71 rear end in a 4000 pound vehicle? is there a formula? (This is a '72 BBody but I trust you guys/gals for some answers) Thank you.