A gearbox is the first thing that popped into my mind. Heck, set it up with a 3-speed manual trans (low buck) and run it in second gear. You could also run it in first gear for low-speed, high-torque. Or run it in reverse to back up. "Hey dude, check it out, my airboat backs up". Build the motor at 9.5:1 with a tight squish and Crane 204541 cam, 216/228, 112, 0.530"/0.560" power range 1800-5400.
For coming right off the crank to the prop, the stock cam will probably produce the best torque numbers at 3000.