I thought the crossover pipe was only needed for engines that had an odd firing order. Since the 3.8 v6 has an even firing order, I didn't think a crossover was necessary. Of course this is based on a forum I read somewhere.
If i can find where i read that, I'll post a link to it.
Edit: Here is the link -
http://www.miracerros.com/mustang/pipes_hpipe.htm
Edit #2: I will be sticking with the stock 2.25in diameter pipes and add turndowns right off the muffler.