if a pin won't hold it.. glue won't either..
The splines are a one time deal and serve for alinement only. There is no movement of the shaft in or out. You have a double roller chain that connects to geared sprockets on each of the rodded pieces. Your splined piece comes off the front crank of the engine. Drill and pin it if there is enough room. If not stuff your coupler full of the loctite and bolt everything together-including hooking up the double chain coupler. Any small amount of wear will be to the chain coupler. Length of the overall piece is critical. This is factory on the early 245dt,s. Later ones use something similar to a constant velocity joint instead of the chain and gears.