The reason I did this is that it's about 14 inches from the main tank to the carb, so if all that line starts out empty, it takes a lot of cranking to fill it. You wouldn't want to plug the muffler and crank the engine over to prime it, because a little too much and you've flooded it again, so the header tank is very close to the carb, with only a short line between the tank and the carb. If you use an electric starter, it will start pretty quickly without flooding (remember that carb is upside down, and excess fuel won't just run out). The main idea here is to have both tanks pretty level with the needle valve. If the tank(s) are higher, as in the case of the stock tank location, fuel will siphon downhill and that's trouble.
Mount header with clunk pickup outlet level with carb intake.
Plumb from clunk pickup on the main tank (that used to go to carb) > to the input of the header (on top).
Plumb from the clunk pickup in the header > to the carb. Put at T in this line for fueling, and a clamp between the T and the carb.
That's it. Leave the pressure for the main hooked up however it was.
Now, when you fill the tanks, you'll be filling the header tank first, then the line to the main tank, then the main tank. You have fuel readily available to start the engine.