Without anything solid to back my instinct, the spray paint does seem to show a lot of holes in close proximity. Swiss cheese isn't a great structural material....
My next instinct is that if the "correct" size pulley can be fitted to the 2 hole pump and everything else fits I would prefer to accept the slight inaccuracy of the 2 hole pump rather than that much adaptation of the cylinder block.
The commercial adapter plates are intended to fit the 2 hole 216/early 235 water pump to the late 235/261 block (pump impeller within the block, not mounted in front of block and head. Do you have a set of the instructions for the 235/261 commercial adapter plate? They inlcude a table of recommended balancer and pulley sizes. A copy of the instructions can be seen on Phils site at
http://www.canadianmilitarypattern.c...ifications.htm