Can't get slip yoke all the way into the trans

[frdnut]

I just changed out the 3rd member in the wifes 65 F100...The new gear setup has the longer yoke at the differential so the yoke at the trans needs to go into the trans further...I figured this should be no problem as it was sticking out about 2 1/2 inches with the old setup.....I cannot get it to go in the trans as much as I need it to for proper clearance and I cannot figure out why....I have another yoke that is similar but about 1/8th of an inch shorter..It will easily bottom out into the trans even with a rusty last inch or so of shaft..
I looked into the yoke and the splines appear to be straight and clean..Why won't this thing go further into the trans?..It sticks out about an inch and half...If I try and force it in it sticks and I have to tap it back out with a hammer..My mustang yoke(which is much longer) slides in until I can physically feel it hit something inside the trans but then it slides out again easily..The length that it goes in is further than the original will go in as well..

[Tech]

It sounds like the inside isn't fully machined out
I might make a stick to use as a feeler to check for a diameter difference.