Lunatic Fringe

I am considering putting extension boxes on an existing side by side, plug-in speedmaster setup and was wonder if anyone has any creative short drive-line options. I don't really want to move my motors forward, this year anyway. What is the shortest traditional drive shaft length I can go with considering two flanges, two u-joints and some sort of slip yoke? Is there any sort of coupler out there that can accomplish the same thing in a shorter length?

Thanks
Jim