I don't know what "FI" is. But if I had the money I would do the long tubes and have the cats welded in by a custom exhaust shop. I wouldn't put a single stock part back on other then the cats, and might buy new cats based on miles. The weight savings alone should net you some MPG's especially because those stock exhaust manifolds probably way a ton. Going true dual exhaust is up to you. I'm not sure I would spend the money on that because its a truck. Now a muscle car I would spend the money on true duel but that's just me.