Funny enough, I just did this to my Magnum block/ speedmaster head build. Don't mean to hijack just figured I would show how I did it.
Started with a 1/4NPT tap at the hole, then used a 1/16 drill bit to break through into the rocker stand. Unfortunately I didn't get any pictures of drilling out the mounting bolt but I did use 1/16 up the shaft and slightly smaller for the cross hole in the bolt.
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Connected the 2 using small -3AN break lines from allstar
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