Oh i feel for ya man ! But before you rebuild the rebuilt motor ... carefully check those valve covers !! .. lots of times the oil will trickle out of the back corner and down the side of the block and end up looking just like its coming out of the main seal (been there done that!) ... also the rear gasket on the intake will do the same thing.
I'd do just what BBCoupe said and clean it up really really good then start it and watch it closely from all angles
If your you're building enough pressure to blow a main seal with two breathers then you have a serious ring seal problem !