The fans are a big power draw... but you have dual batteries and a decent sized alternator, right? It shouldn't be drawing more than 30-40amps total as they run. The GMT400 serpentine alternator I have for my 454 is a 110amp, the stock 80's one is like 40amp or something silly. How do you have the second battery connected?
Is it on an ignition activated relay with a large 4-8ga wire? How are you feeding power to the fans?
There is a solenoid that mounted on the front of quadrajets to kick up the idle when the AC was activated, that might be an option.