Driveshaft length using 1350 transmission and pinion yoke?

I bottom the slip yoke on the output shaft, pull it out 3/4-7/8 and measure.

I also don’t use the long seal on the tail housing. Never found the need to cover the slip yoke if it’s not out a mile.

You want the most spline engagement you can get. That means bottom it out, get it where you want it and measure.