Get the gates belt that they have, Its called a Synchronous. If you buy another belt, the thing you need to make sure of is that it's not a Modified Curvilinear. Gates salesman will gladly help you out with your issue with atleast a part number, then google that number.
You might try this... go to the dealer, ask the parts department to bring out both style belts. Make sure to get the part number off the square cog as you say. Once you have the CORRECT part number that is for your truck, google the part number. You will get several hits on the part number online. Find the cheapest priced and bada-bing, there you go. If you have a piece of your old belt or the old belt, look for a number on it maybe. Bring it to the dealer for example, some of them part guys only know software and were not even born in 1989, lol.

The parts are out there, you have to do some searching...RockAuto can help too. Click the details on the items you choose for your truck and then call the company if needed. Don't give up and don't listen to some tird head on a phone that says there is no such thing either.