Dan,

You will want to use some sort of dogbone or cardan style u-joint. This will help with vibration as well as any mis-alignment that the motor may have in relation to the shafts. FYI: I built this boat some years ago and powered it exactly the way you are powering yours only I used 25's. You will not be sorry you used the Astro-Flights. They hum along at a very scale-like speed and sound quite a bit like twin diesels. I used cardan style u-joints as a point of reference but if you use the dogbones I am sure they will work quite well too.

Best regards,
Pete