Edelbrock's T.E.S. is awsome. (Tubular exhuast system) a little costly but I have put a set on my Camaro and loved it. I then put a set on my 4.3 S10. Really liked them after that.
I am going to get the set up for my future 89 K1500 soon. Buying the truck Friday. $431 for the stainless and more for coated.
The headers on most set ups come with a Y pipe and they make it simple to connect to your stock convertor. Its a do it yourself add on in most cases.
From what I read they dont void any warranty.
The folks at Edelbrock.com will answer your emails FAST too. So I would just go there for any info you can rely on about T.E.S.
Flowmasters are known for that interior sound you are getting. You will love it or hate it. I cant drive with out it.
I like to put turn downs after the Flowmasters.