I agree 50 amp a touch on the small side, though it does depend on what load you're expecting from it. If, for example, you are discharging the batteries by 40-50% overnight or between charging, it probably is too small. Again, as others have said, if your 'operating load' i.e. load during charging, is also high, it adds to the problem.

As I said in my previous post, get the alternator checked out. Lucas did it for me for nothing when I popped in, but it only takes a few minutes on a special rig, and even a few pounds may save you a hundred!!