Split bore (hopefully not)? Turn motor over to expose all of #5 cylinder. Have real close look at cylinder walls.
What's your engine bored out to? Have you been flogging it
Possible head cracked like you mentioned.
Was the cylinder full of water before you pulled the head? or did the excess water in the head/block fill it up when you removed the head? It's not going to turn ever with a full cylinder of water..........bending conrod stuff.
Check the head surface for being square. Is it warped? Was the head torqued down OK?
If it did fill up as you removed the head.........check the timing chain cover plate behind the water pump. They can rust out with microscopic holes and under pressure water will be forced into the sump.
I'd say it's possible to get some water passed the rings..........especially if its been sitting a while. It doesn't take much water to turn the oil milky (been there
Is there any water up in the valley/lifter/rocker area? ie: have you been pumping milky oil around the engine for a while?
Hopefully its an easy fix for you.