Again I am not as familiar with Chev as GMC. Another place where GMC is very prone to crack is in the block behind the push rods. This comes from freezing rather than overheating and blocks are seldom checked there for cracks. I take the push rod cover off and pressurize the system. It's not fun on a GMC because its a tight squeeze between the cover and the distributor and the oil filter is in the way. I still think you have coolant in your oil and that's not good. I used to block off the water pump hole on a GMC block and fill the water jacket with Diesel and let it set for about a week before I built on a block. Diesel will find small crack and it leaves an oily spot.