318 Dis-assemble & Assembled 1st time

Just to start with The vent cap on the pss side let's the oil fall back down to the oil
pan and if it was pluged up or someone put after market valve cover's on and did not vent the oil would hold on top and your valve seal's let oil in your combustion chamber.
see if the drain hole's are restrickted and this would slow flow of oil down out of your head's.I would have the head's cheacked and cleaned.A bad valve will give you low compretion,How did your head cascket's look when you removed them?
I am sure someone will get to you soon with more exp.