The rack fitted to your car is almost certainly a Cortina III/IV/V rack. The Sierra rack might fit with a lot of bodging, but be sure to check that it is for leading steering arms as in the Granada uprights, or you might just find that the car goes left when you turn the wheel to the right!
The most practical way to put PAS in your car is to fit a motor assisted steering column, as fitted to Corsas, Citroen AX, some Peugeots and other cars. They sell for around £50-£70 on ebay, and can be set up to vary the amount of assistance by sensing the input force, very clever!
I have explored this avenue extensively, as many rally guys use them on their Mk1 and 2 Escorts, and as they are electically powered, they don't sap any engine power. I have several different types in my workshop, so if you want to see what they look like, you are very welcome.
I've also got a Sierra Cosworth rack lying around too.
Mike