Heads didn't set correctly? That was my issue. When I rebuilt my olds. It would be fine at idle (no water in oil) but as soon as is give it some gas. The water would make it's way in. I also did a compression test (all checked out) as well as a leak down. Held 20lbs over night. Eventually pulled the heads off, had them re machined at clay smith ( previous machine shop was a joke) and currently have 5 trips with no milk. When you put the heads on, did you spray the head gasket with any copper spray? And made sure you did the toque sequence (3stage) correctly?
Sent with my Fingers