Do you plan to actually drive the truck, or are you building a trailer queen that will only be driven on and off the trailer at a car show? Headers of any kind on a street-driven vehicle are a huge mistake- - - -they leak, burn out spark plug wires, and increase the under hood temperature dramatically. Get a set of rear-dump exhaust manifolds from a late-1980's 350 and you'll be much happier with the whole situation. I ran those manifolds on 400+ HP round track engines- - - - -real dyno figures, not bench-racing fantasies after consuming a case of Budweiser!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!