I drilled and tapped many of my own forged alloy stems for this in the past.
Never had one break.
After doing the first one "freehand" I learned to always set it up with a clamp and drill press so the hole and countersink was straight, concentric and at the proper angle: measure thrice, drill twice, tap once.
The slick trick I chanced on was to insert a small length of teflon cable liner thru the adjuster and stem: worked a charm.