small block 340 driveline in a big block 727

Later models (dakotas etc.) With the newer variants of 4 speed autos use a yoke that has the same diameter and number of splines. It looks the same, but the splines ars cut with a different angle on their face. If you try to use one of these yokes on a 727 it will be exactly like you are describing. It could be forced in but will definitely not slide and work as a slip yoke is supposed to. They are easy to mix up....... I have done it, I bought one because of its 7290 u joint size, thinking it was the same. I would guess this is what you have.