Alternator reccomendations

A radiator fan will be the biggest load, and you probably require that w/ a big block. I would start w/ a 70 A square-back (80-90's cars & trucks), since they are cheap and easy. Put it on a charger once in a while, at least every weekend, and see how long it takes to top off the battery at 10A. If <20 min, the alternator is probably keeping your battery charged and the charger just set slightly higher. If it keeps the battery charged, it is doing its only job. Many people don't seem to think that.