[spdbrake]

If you do go to a 1 3/4" headers the flange area will be choked down to what you currently have due to the flange bolt spacing. There are a few head manufacturers that make alternate header flange bolts spacing, Trick-flow, Dart, Blueprint, and more.
But even if you change heads and flanges you're still stuck with the diameter on the side pipes. Same chokepoint, just further downstream.

Also it looks like your #1 cyl aft header bolt is loose in pic#2.