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!

You will want your O2 sensors in the same location as factory. My 1996 Suburban (as most did) has real factory dual exhaust up to the muffler. It has four O2 sensors, at the manifold and after the two cats.

As far as tail pipe sizing, larger is ONLY better if you are running at high rpms normally. My former 79 Power Wagon was a mild built 360 running long tube, small diameter headers into 2 1/4" exhaust. This was chosen specifically for low-end grunt, and the truck ran out of steam at just under 5000 rpms (which I was fine with).

Fuel mileage? You're joking, right? LOL! The ONLY way to get good mileage in a Ram of that era is to never start it up!

I can't answer your question specifically, but here's my experience. I have an '01 Ram 4X4 with a 5.2. In an attempt to improve fuel consumption and performance I put on flo tech (EDIT: shorty) headers, a high-flow catalytic converter, and 3" single exhaust from the cat back. The mileage was no better, and if there was a performance gain, I couldn't tell. After I developed a few exhaust leaks (over a couple years), I traded the headers for stock cast iron logs.

If my fuzzy memories are correct the "manifold replacement" stainless steel Edelbrock headers were tested in the late 1990s to yield about 5 rear wheel horsepower at 4500 rpm wide open throttle. Their primaries tubes were variable from 19 to 26 inches long.

38 to 42 inch long primaries gave better hp gains.
ID from 1.5 to 1.875 all yielded about the same.

The Magnum V8 beer barrel intake manifold runner area and 15.5 plus 3.75 inch cyl head port lengths "resonate" around 3800 rpm, so header primary length will yield best gains when it too resonates near there.

If you measure stock exhaust backpressure at typical highway cruise it is about 2-3 psi.
Dropping it to 0.5 to 1 psi will not give much fuel economy gain, maybe 0.3 MPG more at 70 MPH.

Full throttle 5000 rpm backpressure is about 6 to 7 psi.

Ok, I've been through all of this except for headers on my '01 360 Off-Road, so, here goes:

Cut your convertor out. I picked up around 1.5 mpg and it downshifts less on hills. Best, cheapest (free), and most noticible mod I've ever done, bar none. PM me and I'll tell you how I fooled the downstream O2 sensor.

I had a single Flowmaster 40 which sounded awesome and lasted forever. I recently went true-dual exhaust looking for more power and mileage.

How did it go? I upped the pipe size and went with a Magnaflow copy, zero-backpressure system. Result? no gain. Actually lost some bottom-end power and it got moved to the mid-range which is useless to me. Waste of money, but I got a smokin' deal on the dual exh setup brand-new in the box from the local classifieds.

I did re-locate my upstream O2 sensor to the driver-side manifold and as close to the collector as possible. Reason, because my fuel mileage actually went down and I concluded it was because of lesser back-pressure that the exhaust was too cold and not giving a true stoich reading. After I put the sensor further up the pipe my mileage came back.

Headers? Garbage I think. There is a '98 Ram in the Autowreckers near me with a set of headers on it and that cheap, aftermarket junk has cracks in the center flanges (1/4" metal ). Draw your own conclusions, I could buy them for $25.00 but I couldn't be bothered with that crap. If you spend all day at 7000 rpm in your Ram and need that extra 15hp, knock yourself out, but mine spends all it's time at 1800 rpm and it would probably cost me power at that level.

Hope that helps, real-world experience, not theory. Can attach some photos of my system if it helps you out.

I just finished building my 97 dodge and got the v8 running in place of the v6 it replaced. When we were putting the motor in I thought of maybe doing headers this time but decided not to. Most are junk and they leak and only look cool when you're broke down and the hood is up so I opted not to bolt any on. Last week I had the exhaust done dual 2.5 inch from the manifolds and used flo pro super v mufflers, put the 2 sensors into the passenger side bank pipe this time instead of one sensor in each pipe. I don't think it makes a difference but that's just how it worked out. I been driving the crap out of it, runs great and getting just over 25 mpg us with it, this is with the 5 speed nv3500 trans. This is the 3rd 2.5 inch system I've had built and I wouldn't try anything other than different mufflers for these magnum powered trucks. I don't use a simulator or fouler or any tricks, not saying they wouldn't work, I just don't think they're actually necessary. With this build I wired the harness myself and did away with a couple things I figured I didn't need. Like the evap system and ac.

Thank you everyone for your comments/information. I think that I will just leave the manifolds on and eventually I will have to replace the factory style "Y" pipe with cat and then I might go with a cat back duals.
I am glad to get your feedback so I didn't spend all the money for nothing.
Thank you all again!
Gary