I went 2 1/8 by 30 inch with a 4 inch collector, but you might go to 2 1/4 and have as good results, seeing as you have more cubes. If you are having to bend the tubes down right after the flange, might be worth going to 2 1/4. Mine go up instead of down.
Now that i reread the post, definitely 2 1/4 for the big motor. I doubt you will take much away from the 572, especially if you run a loose converter.