I have a 1999 GMC Sierra w/ a 4.8 liter engine. I am looking into getting headers for my truck and found some from Dynatech. They make a complete system with headers, converters and y-pipe, but they say it fits trucks with 4.8/5.3 liter engines from 2000-2004. Does any one know if they will work on my 1999. Also does any one know what kind of gains you can expect from a setup like this.