I do much of my own work, but have never totally rebuilt an engine. I'm restoring an '83 C10 which threw a rod thru the pan, so I paid a VERY reputable mechanic to rebuild it. Within a couple of days after we got it buttoned up, I drove it a short distance and at one point it idled for awhile in 90 degree weather. I suddenly heard it "spewing", looked at the temp gauge...270 or more. I poured water over the radiator long enough to remove the cap (still "got me" tho). The rebuilder came over and we were going to try to diagnose the overheating problem. He accidentally pulled the dipstick and yep, water in the oil!! I removed the manifold and spark plugs today and see nothing odd. I also had a friend crank it over and watched the spark plug holes for "spitting" water.
I feel like I added water while the engine was still much too hot???
Can someone please give me input based on experience? Can a cracked head cause this condition, etc.?
Thanks for any help...sorry for the lengthy post.  

Yes a cracked head or blown head gasket can lead to water in the oil. All depends on where the leak is. Which manifold did you remove?  

<<<Which manifold did you remove?>>>

Intake  

I was going to suggest you use a pressure test device to apply pressure to the cooling system. Then see where the water is going but with the intake off, that is out. You may as well pull the heads now and have them tested.  

I'm restoring an '84 C10. i bought it off my brother. same problem, water in the oil when he over heated the engine and cracked the heads. he had a mechanic replace the heads and water pump for him. the engine is now being converted to a 383 because when the guy did the work for him he didn't flush the block to get all of the water out of the oil journals. long story short, he spun the rod bearings and i got a nice ride for 500$