I have a 261 block in my 57 Chevy truck. It was bored .060 over. I used the 848 head from my original 235 & milled it down a bit. I did have to drill the extra holes for the coolant to match the block. However, outside of the trepidation of potentially destroying a difficult to find head, it was really no big deal. So far I have excellent compression on all cylinders. It is a big difference from the 235. This is a fairly straightforward low-cost (for this type of engine) proposition that yields excellent results.