Back in late 70's to improve gas mileage on a bone stock 302 Econoline van, I installed full length headers to haul my crew around on a Dickie Betts Great Southern Tour around the country.
Have no idea of the HP gains, but picked up a couple of mpg. and that was worth it.

Heat from the header is probably the reason for the seal leak. Make a heat shield for the steering box, wrap the header, or both.

I have two identical small blocks in two identical E-Bodies, other than one car having long-tube headers: The one with headers definitely has more torque.

There are better solutions to the issue, especially because you don’t already have the manifolds. That being said, I know cars running 11.0’s with 6 pack manifolds.

Here is a short, interesting video making a comparison between a manifold and a header:

www.facebook.com/reel/2809710109189074

Is this a valid comparison that has any application in the real world?

That is probably the scavenge effect from the collector. Not sure what a head pipe do for that.