Don't do any drilling on a cylinder head. If you need to modify something to mount the sender, drill into the intake or get a thermostat cover with a fitting option already built into it. .
Why not drill the cyl head? I have done it to several heads to use a mechanical temp gauge in place of the electric sender. Just enlarge the existing hole. Drilling to intake does no good, we are talking about a slant six, not a V-8. No water in the intake manifold. The t-state housing option has the problem, that the temp gauge will not show the temperature, untill the T-stat opens. If the stat does not open, the engine could overheat, and it would not show on the gauge.