8 3/4 green bearing a body axles fit

I agree the pin could be taking up the needed space to seat the second axle.
You wouldn't need it in there with non adjustable bearings. If that is the problem don't force it by tightening flange nuts. Could damage new bearings. Another area that could interfere is the lock collar against the oil seal inside tube. I am not sure but it may not be needed with the sealed bearings and could be removed if it is holding you up the 1/4".