Should I leave it Stock?

At 4000 ft you might be in a bit of a pickle.
Your 8/1, 2bbl 273 is feeling like a slanty would at sealevel.
All the bolt-ons you can name cannot solve your problem; except supercharging.

Is it possible you have an incomplete idea about headers.
Firstly headers are tuned over a very narrow rpm range, usually working best at the typical torque peak in the mid-3000s.
But they are inertially tuned. They need a slug of hot exhaust gasses moving down the pipe at high-speed, to perform their function. In your case, this presents some problems;
1) because of your altitude, and your very low effective compression ratio, there will never exist a slug of hot exhaust gasses moving down the store-bought header pipes at high-speed. You will get a warm slow-moving loosely knit doughy thing, and
2) it won't pick up speed until much later in the rpm band. This does not help you off the line; not even a tiny bit. Furthermore,
3) the 4bbl cannot help you if the tires will not spin, the rpm just stays too low for too long, with the factory lo-po rear gears. It will be ~40mph with 2.76s before the headers start to work.
Secondly, headers have a secondary function, related to the first. Beginning at just before peak torque and continuing until you shift, that slug of fast-moving very hot exhaust gasses, is supposed to help your engine create the low pressure in the chamber, so that atmosphere will be encouraged to start moving into there, a tiny fraction of time sooner. It does this during the overlap cycle, when both valves are a tiny bit open.In your case, the overlap cycle is too short to be effective.
For you; the only advantage headers can provide is a low pressure place for the warm combusted exhaust gases to exit into, so at least the pistons don't have to pump them out. IMO, at 4000ft, this is flogging a dead horse. Considering the difficulty of fitting headers in your chassis, and the meager benefit of them, IMO, you can achieve similar results with HP logs and a free-flowing dual exhaust, with a well-placed cross-over.
Hang-on; I'm not saying headers won't allow your engine to make horsepower; not saying that. What I am saying is that at 4000ft with a 273, it's gonna take a lotta rpm to measure it, and for the cost of them, and the trouble involved in the installation in your particular chassis; to me, I could not justify it. If your pockets are deeper than mine, and you are 20 years younger than me, or have a drive-on hoist, then have at it.

But I gotta re-iterate;
for a 273@4000ft
Your biggest bang for the buck is gonna be a higher stall TC, followed hotly by 30 to 40% more rear gear, depending on your starting point.On the plains 3.55s are considered the go-to gear. At 4000ft that is not gonna cut it.

IMO, your best bet is to, as already stated, mothball that 273, and drop a stock Magnum in there.
As for me; on that Magnum, I would;
>install HP log manifolds, and any 4bbl induction system.
>install very large dual exhaust pipes to very large in/out, 3-pass mufflers, and anything easy to fit after that, with turn-downs at or just after, the bumper. Crossover recommended but optional.
>Convert the serpentine to the 273 V-belts including the waterpump!
>leave the Magnum cam in there, but retime it to not less than 4* advanced,
That is what I would do.
Furthemore; I would;
>convert to an isolated field electronically regulated charging system, and to electronic ignition.
> with that torquey Magnum, I would swap in an A999, loc-up with the 2.74/1.54/1.00 ratios, and with that, just maybe the 2.76s can stay.At least until the Magnum breaks the 7.25.

Of course a supercharger would neatly bypass all the hassles, of this swapping business. Bolt it on a Shazzam yur done. Drive it into the shop Friday night, Go for a Sunday drive. OOps, yur gonna need bigger tires.