What is the structure near the tunnel. Looks like a channel or rail. It's a few inches away from the floor hat channel.

That's the lower frame rail right? https://baddogparts.com/index.php?main_page=product_info&cPath=2_5&products_id=10&zenid=nortjbn6oj65b81ldlkb52u5g0

This is what it should look like as it connects to the front of the firewall.

Normally I would say hit it with some of this while you have it exposed... https://www.eastwood.com/eastwood-internal-frame-coating-14oz-aerosol.html

But you should probably just replace it ha.

I think he is reffering to the rail just in upper third of the pic.. thats a strip that holds the brakelines..  Man.. you must like that car.. i would bring it to the wreckingyard, after stripping it..  There is a massive amount of welding needed.. pff..  🙈

Do not forget to brace the chassis/car when you take big parts out of it, otherwise you end up with a car where no door or other part will really fit!!

Anyone make that brakeline channel

I doubt it but KFvintage might make it for you