Fan Belt PN or Length

Anyone running and electric water pump on a big block know an autoparts store part number for a belt that would only fit around the crank and alternator?

Or a length?

Any help at all????

Someone has to be running one.

Run a length of tape around the pulleys and then mark it and take it to the parts store. Ask for several belts that are close to that length. Test fit to find the one you want and return the others.