Headers for a A body

First off, what year A body? Early As, no go unless you're willing/able to do fab work.
Later ('67 and up) A body, yeah they're out there. I'll even do the search for you.
Passenger Car Headers | Hedman Performance Group
Are they worth it? They're fine for a mild build or for a situation where you don't have good, or any, exhaust manifolds. You've already got (if you actually get it) long tubes, so why do you want to take a step backwards?
If you get the car, run it as-is for a while as you sort out any NECESSARY work that needs to be done- you may find out that you're perfectly happy with how it performs. Then you can actually drive it and enjoy it as opposed to chasing every whiz-bang, shiny, latest beat-all part that comes along. Do they all help? Yes, to some degree. Will you ever notice it? Most likely not. Does chasing every little gizmo stand a good chance of not only draining your wallet, but will also turn your car into a perpetual garage ornament instead of being out on the road where it belongs. You've said in other threads that you don't want a "project" car- why do you insist on turning it into one?