The extra amperage won't ever be a problem BECAUSE the alt gives what the car needs. You can get a thousand amps alt, but if the car needs just 40, the alt will provide JUST that. If the batt gets at certain point discharged and begin to suck 15 amps, the the alt will provide 55 amps in total, where the ammeter will read just 15 because the initial 40 amps are before the ammeter when engine is running, so ammeter won't sense that. The ammeter just reads what leaves and return to the batt, not what the alt provides.
http://www.dodgecharger.com/forum/index.php/topic,33574.0/all.html
You need an alt able to provide all the juice as posible while iddling, no matter the max output ( of course it is common the more max output you get, the more juice is able to give at iddle ). When you get that, you will find how dark were the old days LOL And how safe will be your wiring system. If you are able to find the iddle specs for an alt and match that with your car, better. The car needs barelly changes between iddle and high RPMs