Driveshaft length

I am trying to determine the driveshaft length for my 68 barracuda with a 904 and 8 3/4 rear end. I have the slip yoke in the transmission and have measured the back of the uj hole (like in the picture attached) to the back of the pinion yoke. Which came out to be 50 3/4". My user manual says that the driveshaft length is 49.27" which is not what I measured. Am I measuring wrong? Or could the manual be wrong?

Thanks