69 Camaro; put in a new TH400 transmission. Bought a 1310 slip yoke. When slipped all the way into the transmission, it is about 1/2" too long. Took the yoke to a shop that had done work for us before. I wanted the yoke shortened and instead he counter bored it so that the splines are now cut 1/2" from the end (no longer flush). If I were to use this yoke, and following directions of pulling it out 3/4", I wouldn't have much of the splines touching and will lose power or worse, right? What happened? Do I need to start over with a new yoke?
Or theorectically - I could have used this yoke and just pulled back 1/4"? 1/4 + 1/2" = 3/4"

 

With the slip yoke pulled back the recommended minimum of 3/4", make a mark on the yoke just beyond the seal. Then slide the yoke out (while trying to turn it) till the splines no longer catch, and make another mark. That's how much the splines are engaged by. I'm sure you're catching by more than you think. How did you determine the slip yoke was too long to begin with, driveshaft length measurements?