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I would recommend against using shims of any kind under the rocker arm shafts ever. This can cause the rocker arm pedistals on the heads to crack, and kill your heads. The shims cause the rocker arms shafts to be effectively larger in diameter, which loads the corners of the rocker arm pedistals of the heads. In effect it acts like a log splitter. Call Ray Barton if you don't believe me. Do it right, and order the right length pushrods instead. New heads cost a lot more than new pushrods.

this is why the shims need to be eccentric. just sticking anything under the shaft can crack the rocker stand. i've never had a problem but i use aluminum eccentric shims and don't over torque.