[RCW]

I suspect two different problems with your 550. The loss of forward when wet is likely because of a missing drain vent on the bottom of the torque converter housing. Take a good look underneath the housing and I suspect you will find either the vent is open or the protective valve is damaged and open to water.

The stalling problem is most likely a dirty carburetor, but check the air tube that runs between the valve cover and the top of the intake tube that attaches to the carburetor. Many times that tube will crack or come loose from one end or the other and that will cause low speed stalling. The other common problem with age is the long studs that hold the intake tube to the carburetor and the carburetor to the intake manifold come loose and allow the manifold gaskets to fail and leads to vacuum leaks that cause low speed stalling.

You are on the correct track to get the service and repair manual, it will provide you with the information you need to correct the problems.