how much horsepower

If the stock 351 gives a peak of 240hp at 4600 rpms, how much would it put out at 2300. Assuming no modifications are done to the engine, is there a formula used to figure it out, or do I need to cruch the numbers thru a Dyno program?

For a crude backup to a fuel guage. In my truck, I use the odometer and in the aircraft we use basic fuel consumption chart /drag charts. It may not work, but my thinking is based on an engine needs .5 lbs of fuel/horsepower/hour to deliver peak horsepower. Regular gas weighs approx 6 lbs a gallon. On paper I get 240 hp at +/- 4600 RPM. (240)(.5)= 120 / 6 = 20 gallons/hour at WOT. This works out to be more or less accurate in reality.
     Cruising at 2300, I am curious if I could make it to a place on the river and back without needing a tow or extra Jerry cans. Not a lot of refuel options in the wildlife refuge.
     I know there are many varibles but I didn't know if anyone already knew the numbers for a stock engine and standard day parameters. Its trivial but I had some down time and started pondering....