They heads need to be removed and drilled or you'll break a tap. That would be very, very bad. Do you have a tap/drill chart? I like to drill just a hair larger in cast iron than recommended for steel to avoid the tap breakge senario.
I did mine in the car many years ago. Not sure what size tap I used but I did not drill the holes bigger or deeper. Just ran a tap in-with lots of tap oil. Needs not to be perfect, just 5-7 threads. I used lockthight on the steel allen head plugs.