I think 2 1/4" pipes will handle up to 300-400 hp. Your 6 cylinder is making less than 200 (?). Lower back pressure is better up to a point. I am not positive, but for some reason I don't think you should go bigger than 1 3/4". Somebody on this board WILL correct me if I am misleading you.

Always get a 2nd opinion, especially if I am the source of the 1st opinion.