assembly questions

I got several overdrive shafts, I wonder what they measure at. Back in a bit.

Ok I think we need to choose a different reference point.
On these three o/d shafts, the snap-ring groove varies in width by quite a bit (up to .4 mm), But they all have a common measurement from shoulder to the front-most edge of the groove, which is 69.0mm or a hair under; on these three.
By your math, yours are;
66.1 + 2.7= 68.8 which is dang close to 69.0
Johndees measures 2.61=66.3 adding 2.7 for the groove makes his 69.0, or a tic under.
By preponderance of evidence then, a tic under 69.0 (2.71inch) looks to be correct.
I think maybe you just need a thinner snapring? I think they are select fit, but I do not know the range of thicknesses available. The 1969 FSM is no help. You would need a parts manual.

You say your front snap ring is 2.2mm That seems awful thick to my memory. Lemmee go measure a couple of mine. Be right back. Nope, mine are all at 2.2 as well.
your stack-up is;
66.7 (2.626), plus the 2.2 snapring is 68.9 right? and all the shafts measure a tic under 69mm (2.716), which by the math is .1mm=about .003 TO SPARE .....
By any chance is your snap-ring bent? or twisted?
I mean .003 is probably tighter than I'd like to see, but it should be enough to NOT be jammed up. If I was building it, I'd want about .007 to .015 between the shoulder and the third-gear with the gear pressed forward. Again the FSM doesn't say.
The only spec in my FSM is .015 to .030 on the cluster.
So IDK. If you have to cut something, I would chose the hub on account of the gear is extremely hard.

Happy HotRodding ...... lol