There are two sizes of rotors for the AMP (and two different caliper mounts), the larger one fits White/AMP hubs, the other is for the early AMP hub as you have got.
That adaptor is to fit ISO discs to the White hub. The offset from the drop out is different, so you may find the disc wont line up with the slot in the caliper.
What you need is either a large disc and adaptor or a smaller disc with a large disc mounting. the latter will need to be made. ( I can supply the drawing for you, but you'd need to see a friendly local laser cutter.)