I wouldnt woory about shims on iron heads, but I wouldnt do them with aluminum. Jusr order shorter ones. A more important thing then the thread count is how the rocker tip rolls over the tip of the valve. It should start near the center, but towards the intake side, then roll over the center, then back towards the intake side again. Get that right, and you're golden. That's why you use the checking pushrods and trun the engine over a few times to get things right.