[Roadsters] datsun ealry 67 1600 head gasket..Repair advice.

[Graeme]

You have already rec'd some good advice...
My guess for a blown head gasket was close eh?
It appears that cyl #3 & #4 are the only ones with a coolant issue.
They are the lowest ones when you look at the slight tilt of the engine.
Once again, may be due to head gasket or corrosion issues. 
Anyhoo, my advice..
DON'T use an EZY-OUT on the broken bolt!
Whoever named them that was one sarcastic sicko!
Head at least [and maybe block] need to be checked for staightness and
flatness.
Keep the head gasket for the repair shop to inspect.
Head may also be cracked if engine has had a coolant failure any time in the
past, mine was!
I, like others, hesitate to put something like that back together with just
fresh gaskets.
It may last, but the nagging thought of the inherent liability will spoil
any long trips.
I would take this opportunity to pull the engine and re-build the whole
thing.
A fresh top end is really efficient in trashing a tired bottom end!
Oh, and due to dodgy Financial dealings in a country which shall remain
nameless, money is an issue for EVERYONE at this time!!!!!!!!!!!!!!
I have even considered going back to work, but only for a millisecond or
so!!
Got over that unhealthy thought VERY quickly!