I changed out the mufflers on a SPF with an FE 427 recently. He had pretty restrictive crushed pipe mufflers prior and can roll a baseball through the ones he has now. He says it's the single biggest performance increase he's done to the car, I also noticed very little volume increase. There's a good article about changing the mufflers in Kit Car a couple of issues back. If you need any further pointers on it ask here. We got the mufflers from
http://www.stainless-specialties.com/products2a.htm