These guys have some know-how about this swap - http://www.discbrakeswap.com/body_index.html and may give you some straight answers over the phone right away. Don't get me wrong, there are many others with similar products and know-how in the business that can help you. According to the above source, you can use wheels as small as 14" with the small rotor version of the Granada brakes - but........unless you have the later model disc brake wheels with the wheel center relieved (pushed) outward, the calipers may drag in that direction (toward the hubcap). I don't believe the 15 inchers are immune to this same situation. It could be that a 1/8 inch "sandwich" spacer will do the trick - but I think a guy would have to try that one on for size.
Also - and Chuck will vouch for this - the lower ball joint "pin diameter" on our fifties cars is larger than the Granada. The lower mount hole of the spindle knuckle either has to be reamed up to the larger size or you need to use the Lincoln Versailles knuckle that already has the larger pin.