I'd be about ready to drill a couple of holes through the assembly and press in a couple of stainless shear pins to keep the jackshaft and output shaft as one unit... The vibration caused by solidifying the shaft that way would probably outlast the splines anyway.

http://www.bnmotorsports.com "And I will strike down upon thee with great vengeance and furious anger those who attempt to poison and destroy my CEG brothers. And you will know I am the Moderator when I lay my vengeance upon you."