I realize this is an older thread but since I have been researching this topic and still had questions I thought I would kick in a tidbit of experience.
I had planned to have my 235 head (*913 casting #) drilled for the addition of the steam holes for my 261 block and had difficulty finding anyone who would do this.
I took a good look at the head to see if I did this myself, what I would be drilling into. I used a small piece of phone wire to poke into the water jacket to see how deep the cavity went (This cavity is parallel to the milled head surface). The steam hole closest to this water jacket hole could be drilled straight through, to intersect the cavity. The second steam hole farther away, needed to be drilled at the slight angle to intersect the cavity. I read about drilling at an angle toward the manifold... around 12 degrees. I'm of the opinion that if all holes in the head are angled, it is because the head was tilted to this desired angle in one set-up. I only needed to carefully locate the holes so they would match the block... double... no, triple check!
I bought a titanium 1/4" drillbit from Lowes, located my holes and drilled them with my hand drill, with the head on the ground (so I could put some weight into my drilling). The drill depth was around 3/8" and the titanium bit ate through the cast iron, like a butter knife. It was easy and I fretted all this time for nothing. I'm happy that my block will be able to breathe better!
I hope this helps someone with the same questions in the future.
Jerry
