Well after another trip to trade tools to waste more money on tools (inner hex bits, and a decent set of vernier calipers which I have wanted for ages), I managed to remove both the large coolant gallery fittings:
I'll fit the bungs from the other end of the heads in their place, once I get access to them (cam belt has to come off).
I'm not sold on what to do with the other fitting. Its for the heater hose, and the other end of the heater box goes to the bottom radiator hose. I can either:
a) run a hose to the bottom rad hose
b) put a bung in instead of that fitting
Option a) would work just like a factory system with the heater core bypassed, but means running a hose the length of the engine, which is annoying and messy. Option b) would be neater and simpler, but I'm worried about affecting the coolant flow for anything cooled by that gallery. Thoughts?
I also used my new verniers to measure that oil filter hole. It is, near as makes no difference, 18mm. I stole a fitting from an RB30 block, it has a M18x1.5 thread to screw into the block. I am hoping that while he is out here the thread repair guy can either helicoil the hole to that thread, or failing that tap it to a 3/4 UNF thread. I will then dodgy up a -8 AN male-male adapter to allow a relocator block to screw on.