Well, from what I see on the various websites, it should fit. Pictures, as @Coz said, would be very helpful. Does the spring fit ok with no pads in the caliper? Are you assembling it the way it's shown at 11:00 minutes in this video?:I bought remanufactured Motorcraft ## BRCF430 and BRCF431 from Rock Auto. They are identified as 2015+ Mustang PP calipers.
It's possible it's a pad backing plate problem - wrong shape or something - unlikely but possible - shape issues happen with GT350 calipers, although it's not generally a problem.