Gates had two different quality belts we used on the Verizon Fleet. I always stocked the green stripe belts. Some of those vehicles had four and five belts that were a pain in the butt to change. They kept their fleet vehicles up to 15-years so when it was time for belts I put the best ones I could buy to cutdown on amount of time I stood on my head installing belts. The Gates green stripe belts are the best of the aftermarket belts. The notches allows the belt to be for flexible.
We had a mechanic that could change several belts on a vehicle in just a couple of minutes. He stuck a screwdriver between the belt and pulley. Disabled the ignition. Then he hooked up a remote starter switch and flipped the belt right off the pulley. Then he got the new belt half started on the pulley and flipped it back on with the remote starter switch. Some of those vans where terrible to try and get down in there to loosen all those adjustments. I always did it the way you are suppose to because I knew what he was doing had to be damaging the belts. Plus how ever tight it was when he flipped it on there was how he left it. It had to be wiping out bearings. He told me when your at the dealer on commission you need to be fast. I wasn't at the dealer so I did it correctly, standing on my head. That's probably why I've had to back fusions and surgeries on both shoulders.
Save your old belt and cut it. Then it becomes a measuring tool. Back off the belt adjustment. Wrap the belt around the pulleys to the shortest length that you will be able to get on there. I keep a collection of different cut belts for figuring out what I need on hot rod projects where there are no catalogs to look up the application. You want the shortest belt you can get on there so when it stretches you still have room for more adjustment. They do stretch after a short run-in period. Its always a good idea to go back and check them after you have driven them a little bit. Do not overtighten them! If they are not squealing (slipping) they are good.
Remember, on a "V" belt, the sides of the belt are what is driving the pulley. Its very important to get the correct width of the belt and the correct pitch angle of the sides. If you put too narrow of a belt on the bottom of the belt will be driving the pulley which it is not how it is engineered to work.
Here is the Gates catalog to look up vintage Ford belts. Go to page 978 to 981 for the flatheads. Once you have the number go to John's chart above and verify it is the correct length and width. Verify it with your cutoff old belt. Then go to Rock Auto and order one unless you can get it locally so you do not have to pay for shipping.
https://www.gates.com/resources/reso...s-2016-catalog