Mike,
The only exception I know of is 1954 and this is just do to the fact that I have both a 235 and a 261 from 1954. Some of the seasoned veteran bolters might know of more exceptions. I am sure that one of them will chime in soon.
Has he gave you any numbers from the head? Heads from 235s and 261s are different and have different casting numbers. This number is located on the manifold side of the head and is located near the front and on the top of the head near the valve cover. The head casting number could help narrow it down. However, it still not be as conclusive as a block casting number because it is possible to modify a 235 head for use on a 261.
Good Luck I hope it turns out to be a '261.
