the "specific original" clutch could have been 10" - could also have been ordered with an optional HD 10.5", or if it was replaced in the past 50 years, it could have gotten the 11" - very unlikely it would have any other than those 3 ... the 10 spline is correct
considering the fact that you have no assurance that the driveline has never had any work done, the best plan would be to pull the bellhouse and check - the engine is out of the truck isn't it?
Bill
