Fourteen inches is definitely too low. The standard transom was supposed the be 15" vertically. Since the transom angle standard was 12 degrees, the distance along the transom was almost 17". I usually ran a 17" (vertical measurement) transom, putting the "cavitation plate" half an inch below the running water line. Most speed gains on outboards is finding the correct propeller for your boat and load.
However you do have a design problem in your Fleetwin in that the connecting rods have bronze bushings. The factory compensated by increasing the oil in the bronze bushing 5 & 7-½ horse motors. They did it by giving those two smaller 4 gallon fuel tanks. The bigger motors use the 6 gallon tank. Adding the recommended quart of oil gives you a 24 to 1 ratio for those bigger outboards. They have needle bearing connecting rods and run very nicely on that ratio. That same quart in a 4 gallon tank gives you a 16 to 1 ratio to better lubricate the bronze bushings.
When they booted the output to 8 HP, they did it by putting needle bearings on the connecting rods. Later models gained some by tuning the exhaust and improving the combustion chamber. A lot could be done to increase the output of your Fleetwin, but it would be more economical to just buy a bigger motor.
