Blew up 4d55 last night-coolant system overpressurized

Driving home from the junkyard yesterday after scoring cheap Geo Metro parts and my original 1983 120k mile 4d55 made some funky high pressure hydraulic noises and started blowing massive coolant out the reservoir which promptly vaporized on the exhaust and turned me into a freak show of swirling white-gray smoke, much to the dismay of many Santa Cruzians around me. Power dropped off hugely, wouldn't idle without goosing the throttle. While limping into a parking lot, noticed lots of smoke while accelerating which disappeared on snap off throttle and coast.

Rad cap seems healthy, just doin it's job. Overall coolant level didn't drop much, recently topped off. Been dramatically down on power for months now, figured ring seal was shot. Relatively new turbo, shouldn't be the problem. No visible engine leaks or rad leaks. No diagnostics yet, but sure does feel like a rotted head gasket (oem, never touched)

Any thoughts out there before I tear into it?

I don't know. Compression test first, I suppose. Does the oil have water in it (does it look foamy/opaque)?

if it was mine i'd be thinking: if the water pump/thermostat stopped working the coolant may look full 'only dropped a little' at the cap, but wasn't circulating it through the engine too cool it down, maybe never making it past the thermostat housing, look for any signs of it leaking around that water pump, pull the thermostat housing off & see if the stat was working (do the test w/it in water, etc.) --- what did your temp gauge do thru all this ? anything ? or where u too freaked out to remember to look (me, sometimes). Has the temp gauge been reading typically higher than it was before you started losing power however long ago that was? All that pressure could have come from the water trying to circulate, getting over heated, and actually have been some sort of backpressure maybe??? Before you go tearing in to it yeah I'd try to find out wtf went wrong in the first place or else you'll do a lot of work and may not fix WHY It happened in the first place. Take off the hose that goes from the top of the rad to the overflow and blow thru it, did some sediment get jammed in there somehow? Is the hose going in from the overflow cap to the bottom of the overflow container too long & maybe sealed itself like a finger over a straw and didn't let the water flow in/out like it should? (cut mine at an angle just to be sure idk fyi i'm paranoid) You said you just topped it off ~ have you been topping it off a lot lately? more & more lately? or just once a month? Think about if you've been adding more than before, etc. and how the power loss if at all has been related to that.

good luck and yes, what huff said, how's that oil lookin? and how does it smell? lol.