How much room is there from your water pump snout to the radiator? With a big-block it is very tight - other useful fans like the Chevy HHR won't fit. In fact the plastic on the fan shroud has to be trimmed a little to get any clearance at all. Here's a couple of snaps.
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It's been many years since I owned a slant-6 A-body so I don't recall how long they are.