Need water pump asap

I don't think so - it's your engine though. I don't know of any high flow units, there was a Flowkooler a few years ago.

Summit lists this, but I don't know of anyone that's run them, or do I remember seeing anyone talking about them.

GMB North America 130-1070P - GMB High Performance Water Pumps - Overview - SummitRacing.com

Very few threads about high volume pumps too - something else that makes me think a quality OEM model is fine:
http://www.turbobuick.com/forums/general-turbo-buick-tech/337719-coolant-waterpump-q.html
 
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