When I first built the 39 I had a problem like yours.... it was two things... first bad thermostats after several I finally gave up and made a a blanking plate with some holes in it.
Next was 'block filling'...It didn't! I used the sqeeze the lower hose to pump the water through the block.
I've now fit a hose from the back of the head to the thermo housing to let the hot water purk from the back of the head instead of pushing it out the overflow
Your motor is heating up so quickly that you may have all these problems too
Even when you get it right the water will still find it's own level if the rads too full
Good luck
