I would add that a year ago I bought a 261 truck engine from a bone yard in Wisconsin. It was a 62 replacement block (no serial number), drilled for hydraulic lifters, no oil filter, apparently ever though it has provision for the full-flow filter, PCV and the 848 head with valve rotators. The sheet metal was painted yellow and the cast parts painted orange. Yellow engines came in 56 and maybe 57 two tons, I believe.
Most of the reports of 261 heads indicate that they are 848 heads. The US parts books indicate the lower compression 8:1 head whose number I don't have at the moment.
