Hydrolock
 

just finished building my 455. started it up and it ran fine for awhile. then it shut off. Tried restarting it and it was very hard to turn over. Got to looking around and pulled the oil filler cap off and noticed frothy oil and some condensation in the filler tube. Doesn't appear to be any oil in the pan though. Oil is clear and clean on the dipstick. Pulled off the left valve cover to take a look and it looks clean. Just oil. No frothing or anything. Wondering if I'm getting coolant in the cylinders. Stinking valley pan intake gaskets on 455's like to leak bad. But I was extra careful and used LOTS of RTV to get it sealed up.

Also wondering if it might just be a build up of combustion gases in the crankcase. Not using a PCV valve because the GM Oldsmobile aluminum valve covers I bought aren't drilled for a PCV or any other vent. Put a venting oil filler cap on it in the hope of venting the engine that way.

Thoughts anyone?

If it were hydro-locked it wouldn't move. Pull the plugs out and spin it with the starter and see what comes out.
You're going to need something to vent the crank case or you'll be having issues for sure.