Hi Grant
Yes, I've done it both free hand with a hand drill and with a drill press. Go with the drill press. I use a small in expensive bench drill press same one I use to drill frame holes. There are plenty of bolt holes on the front of the engine to mount the table plate to.
Did this when I did the water pump on the 235 or 261 and it was much easier than using hand drill. Big difference is that you can get a straight tight hole that taps well for threads.
The big issue on the adapter plate was the holes that have a space between the face of the block and the machined face of the old water pump face. Hard to get the bolts tight without warping the plate casing a leak. I made spacers so that the adapter plate was completely supported.
When I drilled the block on the 235 to convert it to full flow filter I had the block stripped and mounted on engine rotisserie that time use the regular floor standing drill press, just dropped the work table down enough to get under the engine and lowered the head enough to get with in drill range. Drill slow and use oil on the drill and a sharp new drill get a nice hole that way.
When all else fails use nuts.
Cheers Phil