Hi Jeff,
I also drilled out a 235 head using the 261 head gasket as a template.
I used a titanium bit in my hand drill and it cut through very easily.
I think the bit size was 3/16" but it's been a while. You can measure the block hole size.
As Jerry mentioned, check the passages for any angle you might have to make to intersect.
The critical point is the surface location to assure the hole opening will meet the gasket and consequently, the block holes.
Keep aware of the gasket combustion seal area. The holes are CLOSE and you DON'T want to get into that!
Some people opt to not drill and just let the head cap off the holes in the block... I think that's a mistake.
This 261 bad boy needs to be cooled by design.
Just my 2 cents...
Good luck!
Jerry W.
