Scottly,
Sorry for the delay in responding to your inquiry about the White Bros. exhaust and the 38mm Mikuni.
The exhaust does not use the stock head pipes. The head pipes, flanges and collector are painted, mild steel.
The jetting when I got the bike was pretty far off. Ran great on top end, but very rich in the low and mid range. After some fiddling, ended up with the following:
Pilot-35
Needle Jet-P-5
Air jet-2.0
Slide-2.5
Jet Needle-6F8
Main-220
The bike idles smoothly and pulls strong throughout the RPM range. I haven't welded a bung into the header yet, which would allow me to measure the A/F ratio using a wide-band O2 sensor, but the jetting is darn close.
Good luck.