The transmission is now mounted on the frame, sitting at the body shop...so I don't get to check this stuff daily.
Pretty sure it's not an o-ring because it's hitting metal when the slip yoke reaches the end of its travel. I'll measure the output shaft length next time I visit.
Also I don't still have the old trans, only the pic. I was sure at one point that I would install a 700R4, but I came back to a TH400 and regret selling the original transmission.
Now I'm trying to piece together the same setup I had before.