PLEASE HELP ME FIGURE THIS OUT!!

I HAVE A 67 350, IT HAS 140LBS OF COMPRESSION , AROUND 2500 RPM'S AND KEEPS BLOWING CRANK SHAFT SEAL OUT. THIS HAS HAPPEND 5 TIMES NOW AND I DID A PRESSURE TEST AND THE AIR LEAKED OUT OF THE VALVE COVER. COULD THE HEADS HAVE ANYTHING TO DO WITH CRANK CASE PRESSURE TO CAUSE THE SEAL TO BLOW OUT?

I WOULD APPRECIATE ANY INFO OR SUGGESTIONS!

THANK YOU
JOEY