The 848 head is a 235 head, made from mid '56 to the end of the production run in '62. It is cast with a higher nickel content, to help reduce cracking. It also has the highest stock compression ratio offered for a 235. It will bolt right up to a '261, but like 52pu said the two steam holes will have to be drilled.

-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's