Yep, the head gasket.
I would say definitely.
Travelling a 3000 foot incline, just near the top of the hills the heater core (I guess completely) blew - sending steam directly onto windshield through all vents, whether open or closed on the climate control, meaning I had to open and look out of side window. I did let car sit a while and got to the bottom of the mountain where cellular range came back on and got some coolant in her. But too late.
Symptoms are:
the ignition coil cover popped off around two torx bolts. So heat build up near cyl 5. where something hit, pushed, or bent the plastic. I guess bending of head aluminium head.

Jet of water coming up from expansion coolant tank when turning over. I read this is compression in the coolant system.
Exhaust pipe had steam / white smoke just appeared briefly at the bitter end. Turnover no prob but not ignite. Maybe the exhaust side is hit hardest. Smoke coming from behind engine under or around the turbo area.

Coolant is not in the oil. :}

Maybe it is a rail for the turbo coolant? It is a guess there is a pipe failure because it is like a waterfall back there. A repair shop man who had awesomely come out said he would 'fashion one out of copper or other piping." But also that we couldn't dfeel it but that "there must be a pipe there."

My guess is the heater core was on its way out as I had been hearing noises like a leaf scratching when I used the blower for last three weeks; I even took an image to trace the blower and was going to work on it. That scratching may have been steam bubbling under the carpets near the dash in the heater core. This is basically a worn out plastic baffle through which water has to circulate back through to the engine. I cannot believe the integrity of the engine relies on the passage of this coolant through the heater core.

The gasket was obviously worn and I was also towing min circa 150kg.

Perhaps pressure took out the alum coolant rail somewhere behind / under turbo. There is a lot of water coolant coming directly out from rear-mid engine.

So, there is my ultimate Volvo DIY task.
I think now these cars are becoming classic. I thought I had fixed my car all up - alas this unknown came from left field.

am looking at a 1988 DT? Volvo wagon to replace with but everyone reckons I should get a Toyota

BIBLIOGRAPHY:
The guy who assisted me (click the address and do streetview. This is New Zealand):
https://plus.google.com/112840152969171030216/about?hl=en
the hills Where the steam started: 4 seconds and couldn't see out window:
https://www.google.com/maps/@-37.1556877,175.692564,3a,75y,201.34h,91.3t/data=!3m4!1e1!3m2!1sOnhnOnHXtn_wjYHc0XK3Fg!2e0?hl=en