I used an EB falcon speedo in my cortina. I used the sender out of the T5 box and the wiring diagram off the EB falcon.
It was three wires from the sender to the speedo and power and earth branched into two of the wires the other one was the signal. The speedo has its own seperate plug to the rest of the loom so I grafted that into the original cortina cluster but kept the cortina speedo dial. The dial was the same scale as the falcon one so was easy to swap. I had to chop a lot of plastic out of the dash and make a couple of jumper wires here and there where I had to cut through the green plastic wiring film on the back of the cluster. Another thing to do here is go to the Ford Mods site and find the speedo calibration page. The plastic cogs cost about $10 from ford so you can compensate for the cortina wheels sizes and diff ratio etc.
I would suggest sticking to the EA, EB style dash as the later ones are a different brand and more complex. The oil light, charge light, blinkers etc will be basic stuff. The tacho will be okay also if you use a six cylinder falcon dash in a six cylinder cortina.
The thing that won't work is the fuel sender. Cortina fuel guage works in reverse to falcon if I remember correctly. Something about one varies resistance on the power and one varies on the earth from memory. I don't know if you will get the right scale reading either and you may end up having to ad a poteniometer to modify the resistance range. There are instumentation places around that can do these little mods for reasonable prices.
