Just caught this post BigMac. Cam shaft canbe replaced if there is approx. 30 inches between the front of the engine and the inside of the engine compartment. If not the engine will need to be pullled. It is easier to do on an engine stand then in the Engine well. You are going to need a lot more then just the CAM shaft. If you are the original owner of the rig, you know the history of how it was taken care of. If not, well, your shooting in the dark. From past experience with the 5.7 in boats, gas in the oil can be a big contributer to cam shaft and premature engine wear. A leaking carburatorcould be the cause or a diaphram in the fuel pump leaking.
Like borderline said, a crate motor, may be a wise choice.
PS, Putting used parts in like a cam shaft and lifters, isn't always a great idea. Ya might get lucky, but to do all that work, and then not have it work correctly!!!!! Besides, boat motors have specially ground CAM shafts because of their power band.
Good luck...