Do some reading over here and pay particular attention to page 25.
http://www.borgeson.com/Images/09Catalog.pdf
These guys are industry leaders and know their stuff.
DON'T do what some guys have done like putting a shaft into a u-joint that can just pull right out, yes while you're driving! Splines use a groove, double D use a screw indent to lock the shaft into the u-joint. You can kick out around obstacles if you use another u-joint but it must be supported properly. If I can help with design or look over any pics of what you're doing I'd be happy to help.