Drivetrain Clutch Sizing

So i went to go install my 10.5 inch ram hdx clutch kit and its too small for my car. I took a tape measure and i measured 11.5 inch surface diameter on the stock pp and disc that was in the car. So before i send my wrong clutch back to summit and get a new one, what size to i need? The only other size i can see on their website is a 11 inch clutch whih still doesnt seem right. Anyone wanna help?

Car is a 99 but has an older npi motor from a 96-98 car. I bought a 10.5 ram clutch and it was just too small. I measure the disc diameter and the clamp surface on the pressure plate on it and theyre both 10.5. On the cars clutch it was 11.5 for those dimensions.