Speedmaster spark plug holes improvement??

A long 14-1.25 MM bolt with the head removed may make a good pilot. Or run a 14-1.25 die down on some 1018 steel round stock. You could use DOM over that with a homemade broach cutter fitted to the end and internally thread the other end for a fine thread bolt and a jam nut and so you could set the length. A drill (or preferably a drill press) with a hex to 1/2 drive adapter and socket to fit the bolt head would work nicely to run it. Like you had said, O-1 tool steel would only need minimal hardening (it would continually work harden with the heat involved in cutting aluminum) and would outlast most of us.