I just did a little research on hall effect tachs and from what I found they use a magnet on the object being measured (in this case the crank, flywheel, or crank pulley) and the sensor "sees" the magnet and relays the signal to the tach.
that said you would have to mount a magnet a set distance from the sensor on something that rotates at crank shaft speed.
The 0.5, 1.0, 1.5, and 2 ratios are for mounting the magnet on different pulleys or drives from the crank. For instance, the crank turns twice for every camshaft revolution. Therefore if the magnet was on the cam to get engine rpm you would need the 0.5 ratio so the gauge would read double what the sensor said.
I would think for easy of installation you should look at autometer or isspro and see about the tachs that read off the alternator. I have never seen one in person but from what I understand its basically a strap that goes around the alternator and reads the varing magnetic field it creates with higher and lower rpm ranges.
That or find a gauge cluster with a tach in it and a sender from the local salvage yard, should be a plug and play.
punk