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I used a six inch metal rule graduated to 1/64" to set the cover in place and center it on the crank shaft. That is all the GM tool does anyhow. I don't like using the seal to do this because it will put a bias in the seal position.
I put the front cover on loosely then the balancer, center the cover over the balancer and make sure the bottom of the cover is flush with the bottom of the block before you add the oil pan...