There are two ways you can route the belt. Make sure you route it according to this diagram.
What are you asking the parts dealers? They're only usually bright enough to look things up on their computer and most of the time they get it wrong. The part number for the belt usually is made up of the length of the belt. Use a piece of string and run it around the pulleys with the correct routing. Measure that and try to match it up with the closest part number.
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351W 89 Mustang GT Convertible