Do what you need to do to protect yourself. If I were you tho I would have the calipers sitting in a box in the same condition for a little while. Give time for it to blow over, the right thing to do if you get your money back is to have the calipers returned to him so you can wash your hands of it.
Sux because you'd be back at square one and still in need of powdercoated calipers, but would give you the option to just upgrade to F-Body or GXPs and have them coated. Frankly I plan to just prep my F-body calipers really well and paint them with engine enamel and clear and bake them so they are solid. They are calipers so $$$ to powdercoat them to just have something chip them is wasted money, painting looks as good and is easily touch-upable lol
or run the spacers for the bracket bolts
GXPs run a 13" rotor. 05-06 montes run a regular 12" rotor.
All you should need is
Calipers and brackets for F-body and matching pads
or
Calipers and brackets for a 05-06 monte and matching pads
05-06 monte rotors
if you go with the F-body calipers you need to have the brackets drilled and tapped for the factory mounting bolts to the knuckle.
You could keep the stock ones, I think there is a note somewhere on using either F-body or monte brackets and a stock caliper with a 12" rotor but you don't have the 2 piston calipers for the better stopping power. Just a bigger radius rotor and better leverage for stopping. Up to you.