Tailshaft bushing removal/install

66fs- I am assuming if your slip yoke and tailshaft bushing were original then they are the same tolerance as the standard spec clearances. (Unless maybe the 904 has different set of clearance specs than the 727)
Again I am not a mechanic but I assume either transmissions have many things that can become tweaked/worn a bit and contribute to a vibration at 100 mph. Heck at that speed it could be something not related to the transmission at all.