I fit a 240Z 160mph speedo in my old 1200. Complete with trip odometer. In the US, we only got round gauges in the 1200's and 240/260Z's are pretty easy to find in the junkyard. The 240Z speedo body is almost identical and bolts right in. It's even calibrated the same. On the face, in tiny letters it says "60mph=1060rpm" or whatever. I found the same on the 1200 speedo.
The face however will need to be trimmed a little. It's just a hair too big in diameter. Just remember to cover the mechanicals so the metal shavings don't end up stuck to the magnet. Also, for the trip odometer, you have to drill a small hole thru the gauge housing and PCB. If I remember right, I had to solder a jumper wire around the hole for the turnsignal light.
It's pretty easy. If you have both speedo's in hand, it should be easy to figure out.
I know i've got a pic somewhere.