It depends on the size of the center hole of the Mag 500s. The Granada spindle's snout is a bit bigger than the vintage Mustang ones (1969 and before) so you would need to either enlarge the rim's center hole or turn down the spindle. For later year or repro Mag 500s, you SHOULD be ok since they were made to fit a wide variety of vehicles, but I'm not completely sure. Sorry I can't help more.
14" rims are iffy on granada brakes. I've heard there are some that will fit, but I have Granada brakes on my car and couldn't get my original 14" rims over the Granada brake caliper. I can't say if 14" magnums will work or not, but 14" rims in general are not usually a good fit with this swap.
I ran 15x7 inch magnums on a granada swap, the inner portion of the rim was hitting/rubbing the caliper - I had to run a 1/4 inch spacer for clearance - you might have to run a spacer depending on the size of the rim you are using.
I have 14" rims that appear to be knock-off magnums and i had to shave a bit off the caliper to get them to fit. They would go on but would rub the inside of the rim. Nothing a little filing wouldn't take care of