Longtube headers (Stillen's, Nismo's, Doug Thorely Longtubes, Pacesetter, Powerflow) will eliminate your OEM primary cats (OEM cats are welded to the OEM header). If you are a 2wd 04-06 then you will need to buy 2 cats (high flow preferred for performance) and replace those hollow Resonators in your b-pipes with Cats (B-pipes are the short pipe that are bolted to the OEM manifold/cat). You will then need to move your 02 sensor (which is right in front of the OEM resonators screwed into the B-pipes) behind the cats in the b-pipes. If you are 4wd or an 07+ Titan, then you have 4 cats. 2 will be removed with the install of the headers, the other two are in your b-pipes, in the same place that 04-06 2wd Titans have resonators. You can either simply move the 02 sensors behind those secondary cats, and you will be fine, or you can replace those cats with High Flow cats for better performance, but you still need to move that sensor behind the cats. Just to clrify, you have AFR Sensors screwed into your OEM header, and all the headers will have a port for them, those just go from the OEM header to the new header, simple, its that second sensor in the b-pipes THAT MUST BE BEHIND A DECENT CATALYTIC CONVERTOR to keep your check engine light from coming on. Or you can drive catless and have the light on if it doesn't bother you.

Shorty headers (Doug Thorely Shorties, JBA Shorties, OBX Shorties): You have a few options with these, just remember, the shorties sell for a lower price, but have a more intensive and expensive installl. With the Shorties, you could simply remove your OEM header with OEM cat attached to it, and cut off the OEM cat and weld it to the shorty header and re-install. However, I do want to warn you that their have been quit a few reports of cats failing, even with OEM headers, and the cat matrial gets sucked back into the engine and will destroy your motor. It doesn't happen all the time, but it can. Again, a good High Flow cat (such as Magnaflow or Dynatech) is recommended. You can do the same thing as above, but with a quality High flow and be much safer. Also, you can eliminate that primary cat that is welded to the header/manifold and replace it with a piece of pipe. Obviously you or a muffler shop will have to cut and test fit the pipe to make sure it bolts tightly from the header to the B-pipes. Some even put flanges on the pipe/cat that connects to the header for easy uninstall. The same applies from above about the cats. Also, the same applies from above, you have to have a cat in front of that rear 02 sensor to be Check Engine light free. If you have a cat welded/bolted on to your new headers, then you don't have to move the 02 sensor. If you eliminate the primary cat, then you need a cat in the b-pipes (4wd or 07+ Titans have a cats in the b-pipes already, 2wd will need to buy a cat, because they only have resonators in the b-pipe)

Nismo Longtubes: Thought to be the best designed header currently for the Titan. They have a merge collector and equal length primaries. They are also out of production, and the best way to get some is used from another member.

Stillen Longtubes: These headers do not have an equal length primary or a merge collector, they have a dump collector. They are also priced fairly and can be bought fro Stillen and coated before they even get to you, which is awesome. These headers have been ran on a lot of fast Titans, since they were cheaper and readily available before a lot of the other headers (they were cheaper then the Nismo's).

Doug Thorely Longtubes: These are very new to the Titan seen. They also have a dump collector and do not have equal length primaries, they do however have a tri-y design, which is well known for being a killer design. They come Ceramic coated and could be had for around $600-$700, which is a steal for a coated header, but the price may have risen a good bit since then. Contact Doug Thorely about pricing.

Powerflow and Pacesetter Longtubes: There have been very few people to run these headers. These headers have a 3" collector, which is known to be much to big for our trucks and hurts performance. I would not recommend these headers unless your running some very serious mods or you are FI pushing some serious HP.

JBA Shorties: These are the most popular short tube header on the forums. They are made very well, and are on some very fast Titans. Also, at a cost of less then $400, they are hard to beat if you can do the install yourself.

Doug Thorely shorties: Their are few members running these shorties, but DT is known for good products, and I have not heard anything negative about these shorties.

OBX shorties: Again, not many people running these bad boys, but I know they are popular with Ford guys and I the Frontier guys. They can be had for about $50 cheaper then the JBA's, but since their are few Titans running them, its hard to find any info on their quality, but I'm sure it close to if not on par with the other shorties.