I'll open this by saying I'm not a mechanic so my experience is limited to my own vehicles which would be minibikes, motorcycles, and a range of cars from a '32 Ford roadster to various family vehicles and my '80 L82. Then I'll couple that with being an ME and working for a company that makes mostly electrical connectors. In a nutshell, advice is worth the price charged
The first question is what is the current draw on the fans. I looked for something on this, and the first one I came across said 30 amps. 30 amps at 12 volts is 360 watts which is about 0.5 hp. There seems to be a big debate over just how much hp a mechanical fan takes, but it's all in the range of a few hp, so I don't doubt that the fans when running will need tens of amps.
The original alternator in the car was something like 65 amps. If you are still using the original, i.e. never replaced - never rebuilt, then it will be producing significantly less than this. This I can say for sure since in getting my '80 up and running again recently, the choke light kept coming on which indicates it has too little voltage. A new alternator fixed that. End of experience based car knowledge and now on to somewhat informed speculation.
Since the fans will draw at least 50% of the current the original alternator can put out and since the selection of the original alternator capacity by the engineers at GM was based on what the car as built required with maybe 30% margin, that alone would say you need a bigger alternator. My recommendation would be add whatever the spec current draw of the fans is to the original alternator to decide how much capacity you want in the new one.
As for whether to bolt direct to the battery or make life easy by going to the starter, I think the issue here is that if you go to the starter you may add some resistance to the system. Basically the system resistance will be the wire from the bat to starter plus the wire from the starter to the fan plus the connection between the starter wire and fan wire. Note this last one is more important than people realize - per my first paragraph, I work for a company that makes electrical connectors, so this is advice worth something). On the plus side, since the fan harness won't run all the way to battery, having it shorter may save some resistance. The concern with the extra resistance will be you will have a lower voltage drop across the fan as some of the voltage will be used pushing the current through the extra wire and the connection. You could compensate on the wire side by using a larger gage. One jump in gage, say from 12 AWG to 10 AWG will give you about 60% more cross section which is probably enough to manage any difference with the extra length.
The tricky part may be how you connect the fan wire to the starter. Starters have a very large current draw. And the connector on the fan is unlikely to be designed to carry that level of current. So you need to place it in a way that prevents the starting current from passing through the connector on the fan harness. The question here is does current get to the starter by passing from the cable to the nut holding it on to the cable and then through the stud or does it get to starter by passing from the cable to whatever it rests on at the base of the stud. I've never looked at that.
For what it's worth and for what was paid.
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As it once looked and hopefully will again
Rickh
vince vette
stingrayjim jr
. True about the fan with a thermostat... if it comes with one, or if you add one... I know the one i put on did not come with a thermostat, however, i just wired mine straight with a key on hot wire.... given that these cars tend to run a little warmer than, say a dodge pick-up(due to the air flow from the grill) and that i am here in the south where summer temps tend to get on up there(plus the humidity)... i chose to run mine that way your milage may vary :)