[slowbird]

That is definitely a large header. Not sure if you're trying to get the 80tq that your dad is up or what you're looking to gain. But my honest opinion is it'll take a combination of intake and header work. If the plenum is so large you're having issues on the leave that's definitely not right and you will see gains once that is fixed (I'd bet your shift recovery is bad also due to it).
One thing you could do with the headers is move to a 4 or 4.5 inch collect and lengthen it. See how that works. I'm guessing yours are removable so that would be a very easy thing to test.
Fwiw, I ran 2 1/8 to 2 1/4 4" collector on my 1050hp 511ci.