You have a newer slip yoke which has a different spline angle. There are involute and convolute splines styles and the newer slips won't work with the older trans
You need a older version, what was the part # you have for the slip yoke
what do you mean by different spline angle? can you show a pic or a link that shows that? and what is the cutoff year for those? i have a 67 727 that was missing a yoke and i pulled one out of a 84 dodge van that slipped right in.