Custom duals to y pipe

Made some custom duals going into a flow master y pipe my question is what diameter should I have the head pipes that are going into the y pipe, 2inch or 2.25 inch the rest of the exhaust is 2.25 already flowmaster has 2inch to 2.5 scavenger y pipe and a 2.25 to 3 inch y pipe. What you guys think I know this thing needs back pressure ,
9 to 1 500 Avs 4 speed 473 lift Howard's camshaft 3.55 gears