It seems like you likely have this figured out, but I thought I would just ask that you are sure this clutch is correct. In this other thread:
https://6066gmcclub.com/showthread.php?t=47796&page=4
A lot of people were having trouble with the collar being backwards and the spline protruding on the opposite side from the stock clutch. Sometimes people were installing them backwards by accident because of this. Did you have that problem at all? I have been looking for a clutch for my 65 and was only looking for the Rhinopac 04-528 because it sounded like that was the only one that didn’t have that problem and actually fit.
I’m just wondering what this was like for you, because if it all works and lines up then I have a solution for my own problem, but it also might be contributing to your troubles if it was not something you had been aware of.
Let me know how it all fit together for you.