Trying to drive an AC compressor off the alternator slows it down so much that you won't get much cooling below highway speed. Use a 2 sheave harmonic balancer for a 250 six, and drive the compressor with the front crankshaft sheave. The balancer snout usually needs to be shortened about 1/4" to make the rear sheave line up with the water pump and the alternator. Machine the balancer OD seal surface back the same amount as you shorten the length.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!