An 850 head is for a 261 engine. A 848 came from a 235 engine. When I put a 261 back together out of 2, one of them had a 848 head on it. The previous owner wanted to increase compression by using the 235 head on the 261. Instead the water passages didn't mate and caused overheating and engine failure.
Check the engine numbers and specs with your machinist before you rebuild.
