So how is it done guys....???

Ive heard stories of a mechanical to electrical speed impulse converter ( or something like that ) that you can get aftermarket. Do they exist, if so, where can you get them from and how hard are they to install?
What have you other EJ'd MY & L guys done for a speedo...?
Myself, I have an EJ engine, gearbox & ECU but have kept my stock L-Series dash and instrument cluster.
I'd sure welcome your feedback / advice....

L-Raiser