On the look out for a set of these headers for my gk76. Anybody have any experience with them or know where I could buy a set?

They are really rare, and commend a price. You may get lucky if you buy them from someone who doesn't know what they are. You need the corresponding water pipe to go with it, or get one fab'd.
Join a Jap auction site and get hunting!
Alternatively buy some of the Black Widow ones, and send them off to a specialist for modification. Power should be the same if not better.

What would need to be modified on the black widow pipes?

Removing the welding from inside the header collars, and consider re-fabbing the link pipe outlet to 2", as the std system has around 40mm.
This should increase the top end, but lose some driveability.
If you want to properly increase the power, You will need to re-work the head, and sort out the breathing issues the std bike has.
All Suzuki kit engine parts are designed to work together, ie carbs, cdi, headers etc etc.
I had a reasonably fast gsxr400 in our local racing club a long time ago, and it had std Suzuki 4-1 headers and SP cdi/engine internals.