As I understand it, the American truck engines ran an 850 casting number head. Not sure about the Canadian production engine, as they had some differences from the USA truck version. The 850 head will have the extra coolant passages and slightly lower compression than the 848 which was a late mode passenger car head. There can be some intake valve to piston clearance issues with some camshaft/cylinder head combinations, particularly if the head is milled to raise the compression ratio. Most 848 heads should have those "steam holes" drilled to assure proper cooling with the siamesed cylinder wall pairs in the 261 engine.
Jerry
