Measuring for a new driveshaft

so when I google 69 dart 727 8 3/4 I get 47.81" for length but if I bottom out yoke I have to pull it out an inch and a half to reach that length... and are all slip yokes the same length because I getting a new one of those too. I also got the measurement from output shaft to center of rear u joint to. All they asked for was center u joint to center u joint measurement