Jeff,
Thank you for your comments.
The head gasket is a thin copper ring between cylinder and head. I used copper grease to help seal. I did not check for leaks here as all cylinders were machined at the same setting so each bank of four should be on the same plane.
I will immerse the engine in white spirit to check for leaks here. then try adding silicone to joint faces as suggested .
I did run the engine first with it totally submerged in oil with lower crankcase removed and no plugs fitted at a few thousand RPM finishing at 5000 rpm using a prop starter with it fully assembled. Cylinder bores are polished from this process with most radial honing lines removed..
The valve stem to valve clearance could be increased and was something I intended to do as there is little axual play at present. I did not re ream valve guides after pressing in to head.
I found that if the crankcase had too much oil in there it would get past the each ring and fill up the glow plug with oil. For me I found the correct level was just enough for the crankshaft to dip into and through up the bores.
The valve seating were tested by immersing the heads in white spirit and blowing though a tube connected to each manifold in turn. Air bubbles were seen with little improvement after further lapping. This is why I intend trying stiffer valve springs.
I hope these observations might be of interest to other builders of this engine.