The reason I'm going with the double-ended shaft might be interesting to some people here who might be thinking of doing the same thing. The car I'm converting (Mazda RX-8) has a gear-like thing with a magnetic sensor mounted on one end of the crankshaft, and the ECU uses that to figure out the crankshaft speed and orientation. When people swap engines, they usually transfer that device to the new engine, so that the ECU is (sort of) tricked into thinking the engine is still there and it generates all the right can-bus messages to the rest of the car. I haven't tried this yet myself, but I gather that it's the easiest way to get the anti-lock brake system, the power steering, and the tachometer to work properly, and the extra shaft end gives me a convenient place to mount it.

That hyper9 kit is nice! Glad to hear that someone has experience of using them and recommends them.