I just went from a 10 bolt to a 12 bolt in my 70 chevelle now thw drive shaft is to long and will not fit is this common.

No it's not common. Most of the time the drive shaft will fit both rear ends. Did you change anything other than the rear end?

Mine slipped right in. Make sure that you are getting the slip yoke all the way into the tranny.

Rear doesn't have a 1350 yoke does it ?

Done quite a few 10 bolt to 12 bolt swaps on Chevelles...never saw one where the driveshaft length didn't fit after the swap. :confused:

Had no problem when I did mine.

Just did mine in the spring, fit perfect.