I'm probably going to get a lot of flak for this but I'd go with a V-1 Dr. Q Pipe.

Since you'll be running flat ground and will not need a lot of pulling power for steep hills I think this would be your best bet.

The pipe will spin the motor over 9,000 RPM (revolutions per minute) so make sure your motor is in good shape.

And lastly you'll want to fluctuate the throttle (load/ no load) so you don't seize the piston.

The Q-Pipe should get you over 100 miles per hour if your intake and carb are big enough.