The 261 is a very desirable engine, but the 235 cylinder head will need a little modification to make it work properly. The 261 cylinder walls are "siamesed" together so there's no coolant flow between them. There are holes drilled in the cylinder head to allow coolant to flow from the block to the head beside the cylinders and prevent hot spots. The 235 head needs to have some holes drilled in it at the proper location and angle to avoid cooling problems. If you search the forum archives for "steam holes" you should find much better instructions on how to do the modification than I can provide off the top of my head. The 235 head with "848" as the last three digits of the casting number is the best head to use for that application.
Jerry

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