I searched high and low for a modern piston (coated, metric ring pack etc) for my 451 refresh and there just isn't much out there unless you get creative and use a BBC rod around 6.660ish from what I remember.
Keep in mind when looking for pistons that your desired compression height will be (deck height - connecting rod length - (1/2 stroke) i.e. (9.980 - 6.535 - 2.125) = 1.32
