I'm not sure what GM recommends. I always make them so that gentle pressure on the top of the belt in the middle of the span between pulleys would deflect maybe 1/2 an inch, no more than. You don't want it too awful tight or you'll find yourself replacing a water pump I'm betting.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)