Here is a good reason why Flowmasters are not so good.... air flow!
Flow chart 40 series:
http://www.dynomax.com/products/images/floss_chart.jpg
Dynomax ultrafow X :
http://www.dynomax.com/products/imag...ox/x_chart.jpg
Flow chart 50 series:
http://www.dynomax.com/products/imag...weld_chart.jpg
I had the Dynomax Ultrafow Welded series on my 02 Gt. Sound and performance was pretty good IMO. Dynomax is a pretty good muffler and their inexspensive too. I beleive Autozone and advanced sell them too.
I beleive when fabbing up the H or X pipe you need to find the "HOT" spot in the exhaust and thats where you make the connection.