emmissions regs
Careful about emmissions on '96 and newer vehicles, here in CO its more stringent for these than older trucks. Its actually illegal in some cases to replace / move the stock cats. If its a concern, then you need an approved exhaust system to stay legal.
I'd use a stainless steel header / y-pipe system, as Vortech motors do seem to burn a little hotter than earlier 350s. Sanderson wouldn't warranty a set of there shortys I got for a fj55 / Vortech conversion I did because they were installed on a Vortech motor... Maybe an Edelbrock TES system, and whatever cat back system you like the sound of would work well for your truck... My fj55 was pretty quick with a high performance exhaust, intake and hypertech power programmer.