I am running a dual 11" Audi fan I found at the wreckers with my 351 and have had no problems so far.The Audi fan actually takes less current to run than the original 4 cyl electric fan that was on my car so I am not too concerned about upgrading the alternator yet.
I'm using a cheap Perma-Cool fan with my stock alternator, and I agree with what people are saying about upgrading the alternator. If my fan is on at night with my headlights on the volt gauge is one line above the red at idle and I don’t have any other accessories. I will be upgrading alternator just as soon as I can afford the new MSD alternator.