I decided it would be less work to modify the header.

First, I expanded the flange face for port #6 by laying down a bead with the mig. Then I ground it down, and carefully filed it dead flat. Now maybe it will stop blowing out the gasket.

Next, I sawzalled the #4&6 tubes about a half inch from the flange, and at a convenient spot about half way down. From an old piece of tubing, I cut spacers, 1/2" for #4, 3/4" for #6, and welded it back together, after trimming the cut pieces appropriately.

I added some new heat wrap, then squeezed it back in from under the car. The mods made it almost not fit, but I found its sweet spot. Once in place it has lots of clearance. So do the spark plugs. Yay!