Watch those 336 cams. According to a good source the latest ones were made in China and have now been withdrawn.
Swapping the cam on a post 81 machine is pretty easy. You can leave the oil pump in place and do it from the front. Earlier machines had a woodruff key and needed the flywheel removing. Just take the pushrods out one at a time putting the rockers back in place after you take each one out. Turn the engine to move the cam followers out the way and away you go.