You need to make sure there is no old ,dry grease in the coupler , Brak-Kleen and a old shaft. A coat hanger with a bent tip to clean the splines., etc.
A product called "Free-All", wrenches , spanners , etc and sockets to fit the upper and lower nuts. Loosen the top nut and adjust the bottom nut. Up or Down your guess to start.
Worst case scenario is rusted motor mount nuts. Rig a hoist across the gunnals , raise engine just enough to remove the front mount and take to a bench with torches to free it.. Being a IL6 is a breeze