If it will fit and it's just slightly too thick, file off the back of the caliper. It sounds drastic but you only need a couple of mm clearance and is a common modification over in the UK for AP calipers onto CB35, 450 and 750 racers. Of course if you go too far you will need a new caliper.
How much of a problem do you have?
It could just be that the caliper you have simply cannot be made to fit...what make/model is it?
