The way really is to use half washers. Take a washer about 3/4" OD and 5/16" ID and cut a slice off of it, not quite through the center of the hole (so it will stay on the stud). You'll need so pretty burly tin snips or cutting pliers to do that. Repeat as necessary until you have enough washers to make up the thickness difference.

Works fine, lasts a long time.

Yo should get substantial torque gains with that header if you follow it up with a large, free-flowing system. In what rev range the gains occur depends on which of the three tri-Ys you have.

I would not recommend the current iPd header for use on a car. Might be useful if you're setting up a moonshine still, or something.