There are a lot of theories regarding the steam vent system of the LS engines, and whether to leave the rear 2 blocked off or not. The theory is that air pockets form in the top of the head and can create hot spots. I think it depends on how level your engine is whether you should have them open or not....
I decided to play safe and buy a steam vent kit from the USA which links all 4 vents back to the header tank. I had to relocate the N/S lambda sensor connector bracket that I placed over one of the ports. I just made a new stainless steel bracket to mount it and bolted it to the block with a short M10 cap screw.
This also meant that I had to drill out the small plug that blocks off the port on the rear of both cylinder heads. I used a 3mm drill bit with lots of grease and a hoover to slowly drill out the plug. Once I was down a little bit, I tapped the hole out to put in a M4 bolt. This then allowed me to plug out the plug without causing too much mess!
I gave the water jacket a good hoover, to ensure no swarf had entered in.
I plugged the holes with paper towel temporarily to prevent any further entry of bits. I will provide an update once I have fitted the steam vent kit.