Looks like you have a 1954/55 head and a 1955/1957 261. The head should have had some additional coolant holes drilled into them so that the head would be compatible with the 261. The proper head would be 3836848 also referred to as the "848" high compression head used until the end of the 235/261 engine.
Here's a good resource for finding out what your
casting number means
