I stumled across an alternative to stainless works chambered mufflers. They are called PowerSticks..... See link http://www.torquetechexh.com/powersticks.htm

They make one that has a single in with dual out. What size is the input to the stock exhaust on a 2005 GMC Sierra Z71?

 

Pretty sure it's either 2" or 2-1/2". If nobody else responds before Monday than I'll crawl under the truck and get a measurement for you.

 

The stock exhaust piping from the Y pipe to the tailpipe is 2.75"

 

I crawled under my truck yesterday evening and it didnt look like 2.5" or 3" so I guess the 2.75" is the correct size. I'll go with the 3" in and dual 2.5" out. Thanks for the help!