Those aren't actually blind holes; they're through-holes with plugs on one end. You should be able to carefully knock out the plug, run your tap through the hole, and then replace the plug.

N.B. You may find that your pedal spindles are too long; those cranks sometimes needed special short-spindle pedals so the pedal spindle didn't push the plug off. You could use washers to effectively shorten the spindle if it is too long.