I third the nods to Flowmaster. I've had the Force II system w/ the crossflow muffler on my convert for over 7 years and it's worked well. I had to weld an "s" shaped adapter pipe (between the headers and exhaust) to clear the boxed frame on my convertible but other than that it went well.

The one complaint I have is the flowmasters resonate at low idle which can be annoying at times. The only time I like the resonating noise is when the kids pull up to me at stoplights with their subwoofers thumping and the 'ol 455 idle drowns out their music (smile).