[Mart]

You could use an older, fully populated, cast iron manifold as a guide. You could use a piece of tubing to guide the tapping drill and the manifold itself to guide the tap. Carefully measure the depth of the existing holes and make sure you do not go deeper than that.

If you do all the drilling and tapping with the manifold in place, no chips will go in the engine.

If you use the other methods make sure you tape off any possible openings that chips could enter by.

Mart.