Do you know the amperage rating of the alternator? If it's 60 or less, you're okay. If not, you'll want to run the output straight to the battery and forget the ammeter.

Electric fans add 15-30 amps to the system, so you're really gonna want 60 amps at a bare minimum. Figure it like this, 8 amps to run high blower, 10 amps to run all the lights, 5 amps to run the HEI at an average cruise speed, 6 amps on the wipers and 15 for the electric fan. There's 44 amps right there. Now, hit the bright lights, turn the radio on, put the wipers on high, hit the brake lights and you're edging real close to 60 amps and that's w/o any amps being used to charge the battery.

Most cars are designed with 30-40% reserve left in the alternator with all electrical loads on. i.e. a vehicle with a 100 amp alternator will see a max of 60-70 amps total electrical load (w/o charging the battery).

Now, the nice thing about electric fans, the faster you go, the less they draw. At around 90 mph they actually become generators ('course, that depends on airflow through them of course).