probably the best header you can buy for your application is doug thorley headers.. they will have a header for your 454 K10 too..
http://www.thorleyheaders.com would be the first place to check
muffler preference is really a personal thing but most aftermarket stuff will give you a rumble... Dual 2.5" or maybe even 3" exhaust will probably give the best performance.. being that i dont know anything about the engine or your local smog laws, or your budget, I cant say for sure, but there are calculations you can do to figure out what size headers, and exhaust tubing will give you the best performance..
hope this puts you on the right track.. if you give more info you'll get more help
mike