the Mustangs Unlimited catalog lists 10 belt sizes for '71 351c if you include alt belt. The PS belt with AC is listed as 51 1/2 inches. without AC is listed as 52 1/2 inches so they are very close. You may find this info on their website. I havent checked. I have found errors in vendors information in the past. When i changed my belts i taped a piece of string to the belt and wound it around and then measured the length. Since i dont care about concours part numbers on my car i bought the Goodyear Gatorback belts. The real trick was getting the width just right so the belt fit properly in the pulley. Cant be too deep, cant ride above the sides. I dont think you should expect to get a perfect fit from the "correct number" parts folks because there were so many different belts that the PN info can be wrong. The goodyears run cooler and quieter than oem style also. good luck.
