You wont find a header if you have a b2600i... Trust me, I should know. I have a b2600i and the only place you can purchase a premade set is out of Australia for the Ford Courier 2.6 (same motor, same truck, just a Mazda rebadged to be a Ford) and they are $310, not including shipping, and those of us who have checked into it, none of the places will ship to the USA.
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Other option is to make your own.
i have a 3in aluminum intake tube with spectre filter on the end connected directly to the mass air flow sensor for my intake, the air recirc box right before the throttle body has been deleted,
the charcoal canister has been removed from the emissions circuit, 2 1/4in single piece exhaust from the downpipe (my exhaust is essentially a 2 piece system, stock exhaust manifold is one section and downpipe to the magnaflow is all one section), into a 2 1/4in magnaflow muffler and a turndown tip. no catalytic converter. downpipe had 1in section cut out to tuck the exhaust up to not hang lower than the frame of my truck.
not that tucked exhaust would matter for your 4wd...
I have a reese class 3 hitch and have done quite my fair share of towing with it. i'm pretty sure you'd be good to go with your own setup. please forgive this very old photo, but note that its a good reference photo of how i mounted my tow hitch. i did however flip the actual receiver part to raise the hitch up a couple inches as i sit pretty low...
i did add load carrying shocks which give an additional 500lbs of carry capacity
i hope this can help answer some of your 2600i related questions.