Make sure it will fit
I have a 2000 H340. The propane locker can hold a 5 lb cylinder but the locker lid will not close fully if the cylinder is more than 12" high. I bought a new 5 lb from a supplier on the web but it was 12.25" high and it prevented the locker lid from closing fully. Yes - that 1/4" makes all the difference and there is no was way modify the tank or the lid to make it fit. Hunter says that 4 lb cylinders can be purchased from the Hunter Marine retail parts department at 800-771-5556 x 3015. These tanks are made by a company by the name of Worthington - see link. Both of these tanks are expensive. I can use the 5 lb tank but I have to keep it at an angle in the locker so the lid will close and seal. The bottom interior of the propane locker is gel coat over fiberglass so you should be able to get the tank out with careful force. If you look into the archives, there is a discussion about the locker drain hose filling with water and not allowing fumes to vent properly. The propane tanks will sit in water and rust easily - as you have found. Because of the design and placement of the locker and hose, the hose and the bottom of the tank fill with water, presumable from the stern wake when the boat is undeway. On my boat, I am able to keep the hose level by putting wood blocks underneath. That seems to allow water and fumes to drain.