Yeah I got it all working. It wasn't that hard in the end!
- Fuel gauge: no changes so that wasn't a problem.
- Temp gauge: all Toyota temperature sensors have the same voltage range, so I just grabbed the right wire from the engine loom and it worked perfectly.
- Tacho: I had to purchase a signal booster which converts the square-wave tacho signal from the ECU into a waveform the tacho understands. There were only 4 wires to connect (+12v, gnd, signal in, signal out) so that was easy.
- Speedo: I removed the speed sensor from the Mk4 transmission and replaced it with the speedo cable from the Mk2 transmission. This made the speedo and odometer work as expected, but I needed a speed signal to keep the ECU happy! I acheived this by pulling the speedo apart and tapping into the reed switch built into the back of the unit. This provides a square wave signal (1 pulse per revolution) which I simply connected to the ECU's speed input. Hey presto, it works!
Those are the only gauges on my dash, but I imagine it would get a little more complex if you had other stuff to worry about (eg oil pressure). The warning lights were just a matter of finding the relevant wires from the engine loom (easy if you have a diagram) and connecting them to the correct pins on the dash.
Using the mechanical speedo cable means the original cruise control should still work, since it takes its speed signal from the dash. I haven't tried this yet, but I might give it a go one day. I'm not sure how it will respond to boost though!