If you're rebarreling, headspace needn't be checked against the old barrel. Rather, it will be measured from the bolt face to the receiver face (which should be faced off to ensure that it is true). That distance, minus .002" for crush depth, determines how far to insert the reamer into the new barrel.
A decent gunsmith should be able to give you match (minimum) headspace. The "go" gauge should just close easily.
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