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Take the backing plate off and measure the O.D. is the best way. Or you might find your part number and have someone w/the exact same pump measure thiers. And if its OE.. don't assume an aftermarket impeller will be the same. Sometimes they will increase the diameter or use a vane that is more perpendicular to the flow to increase overall flow rates.
If you have specific Qs.. just ask. I'm sorta an expert on Ford WPs. ;-)