Hello, I have a 1999 Dodge ram with a 5.9 (360) gas engine and I was thinking about putting headers on it. The most common headers that I have found are the exact fit headers (manifold replacements) that the stock style/factory exhaust pipe /with cat and oxygen sensors bolts up to.
I also have considered using full length headers and full true dual exhaust but .... if I only have one up flow and one down flow oxygen sensors....if I put them into only one side of the exhaust..will they work properly? -telling my PCM how to adjust the fuel mixture?
If I go with the header replacements for the manifolds is the factory size 2" pipe to the cat for both sides and 2 1/2" out of the cat a good set up to use for power and fuel economy?

I also may in the future build a 408 to put in my truck, but I am not sure.
Any help would be appreciated..
Thank you!

I'd want 2&1/2. any restriction downstream from the collector flange hurts power/economy. header pressure wave tuning (dia/length) is a function of the primary tubes only (& that is important)