Roy,
The fueling problem on your SMC era Beaver is caused by the tank location in most cases, because the tank is located above the front axle in the center of the coach with two or three 90 degree bends in the filler hose between the filler neck and tank. This fuel tank arrangement mandates a slow fuel flow rate when filling, so using a high volume truck nozzle at full flow is impossible.
If the tips in the post above do not correct you issues, you probably have a restricted vent valve. The vent valve is located at the end of a 3/8 inch hose that is located at top edge of your fuel tank, usually the LF edge. The vent valve is a check valve that lets air escape from the top of the tank as you add fuel. It is fairly easily cleaned or replaced if necessary, but do not remove it because it is necessary to prevent fuel leaks.
Gerald