Driveshaft length - 10 to 13 speed swap

Measure the difference in length between the 2 transmissions. Then just take the old shaft in to a driveline shop and have them shorten it up.

 

Did you already have a 13? I was going to do the same in September but had other things to do first. Plus the core charge was pretty high. Guy said 10 speed cores aren't as desirable as they once were because everyone is switching to 13 and 18.

 

The best thing to do is get the new trans bolted in and everything set and sitting normal including the suspension and then do the measurements. This way you will get the absolute best custom length to the combo and not just a ballpark.
Every truck combo is a little different. There are 100’s of different yoke dimensions and different and odd rear collar/bearing retainer sizes too. Not every combo is uniform across the trans. models/series either. Example. I’ve got two old RTO12513’s. Both out of ‘74 Petes. Same identical “book end” and wheel base trucks. Both trans side by side look identical but one is 2-3/4 inches shorter and the yoke is a 1/2” deeper. Making swapping shafts without possible vibes questionable.

 

Best advice your gonna get all day @SmallPackage.