So I can change the manifold/headers without replacing the whole exhaust system, like exhaust pipes and mufflers and tips? Those things can come on a little after?
How much needs changing depends what all you have there.
From what I recall, your pipe flange at one of the manifolds is chewed up and no longer a smooth flared surface. So that needs to be dealt with, and, may not line up after I too much needs removing.
I dont recall if you have a y-pipe or already true duals. If a y-pipe, then headers cost you now in that you'd need to run exhaust behind it.
You could go with some L69 f-body manifolds, and upgrade to a larger y-pipe, converter, and intermediate pipe that would both sounds and flow better than a took, but, not as good as headers. But it is likely the best flowing stock gm manifold setup for a sbc that is gbody friendly since rams horn style manifolds get interference from the front crossmember/suspension. But that's money, and, if you sought headers in the future you'd be undoing all that work. Still, it's worth a few HP and ifna shop near you does custom aluminized pipes at a low cost per foot it might be worth it, especially if, as i said, that existing flange isn't easily salvageable.
Now... as for the exhaust leaks and soot in the pics you've posted a few times not near the heads, I doubt your stock manifolds are cracked or warped being the issue, so, a remove, cleanup, and reinstall should be an option.
Any headers will require some work on the exhaust. Doesn't matter shorty/3/4/long tube. How much depends what's currently in the car, if you've got true duals already, and what the collector size is vs pipe size under the car, you might be able to cut the existing pipes if true duals and connect the headers to it without doing full intermediate pipes, mufflers, and tailpipes. But if stock gbody exhaust is what's there now, no, it'd be a bunch of work to do any header.