I have a 2003 5.9 that I'm about to do some upgrades to .... Injectors , turbo , head studs , Fass fuel system , 5" exhaust , etc.
Question is , if it would be beneficial to upgrade the valve springs and also go with a steed speed manifold ?

 

Lots of factors involved in determining whether or not you need the springs. Like whether or not you're going to choose to put a bump stick in it, or what turbo you wind up with. ( no pun intended) The FASS pump, and exhaust manifold would be the first two pieces I'd buy if I were buying pieces at a time. The manifold will give you more throttle response and spool down low. But I'm a firm believer in an aftermarket lift pump filtration system for even a stock truck that will never be modded. Good luck with it. Bomb on.