What would you do?

I think everyone who has or had cheap headers in a A-body hate(d) them.
They hang too low and get beat up, etc... High dollar headers aren't much better.
I dont know how much mod work would be needed to get the magnum manifolds in there but it might be worth the effort. ( Switched left for right and upside down fits. Lets call that joke of the day. I dont know if it would fit or not. )
I think your left manifold has a near 90 degree turn before the dump that really restricts flow.
Good luck with whatever you decide.