Here is a comparison of a 235 and 261 head, from the famous Racer Brown/Frank McGurk article in the May 1955 HRM. I have put a couple of 235 heads on 261 blocks, so one should be able to put a 261 head on a 235. There will be a loss in compression ratio, as the combustion chamber volume of the 261 head is larger than that of just about any 235 head. You will need to use a 261 head gasket. All this assumes that you have a 261 head. The steam holes will tell for sure.

Hoyt