Have you actually got a need to uprate the alternator?? If your old one has died it will cost you about £20 to make it like new again......unless your running some mental sound system or have 50 transit batteries in the boot it really won't need it (plus bigger alternators create more drag).

Ive got a standard 55amp one on my cabby, and the biggest battery i could fit in the tray and its absoluetly fine.

You'll probably find you will have to mod the wires for the granada alternator too. Just get a new brush/diode pack for yours is what i would do, cheaper aswell.