UPDATE: The more I'm looking into this, the less likely it's appearing to be a head gasket. This, of course, is good. Today, I fiddled with the following:
1. Pulled all 8 plugs, checked the gap, and poked 'em back in there. Everything was gapped right, and the plugs are so freakin' clean they look brand new (except for the oil goop on the outside from the valve covers starting to leak). No funky deposits, nothing fouled out, no shorted out plugs ... nothin'. Only things amiss with the plugs was that whomever stuck 'em in there before was limp-wristed (hardly more than finger-tight) and they used friggin' Autolite Platinums - those will be coming out and getting replaced with Motorcrafts or copper Autolites sometime later;
2. Rerouted all plug wires so they aren't laying all over each other and aren't laying anywhere near anything hot or anything they'll be tempted to use to short/ground out;
3. Found ANOTHER vacuum leak, but I dunno WTF it goes to. Seems to be A/C related in some way because it comes off a T-connection/check valve on the passenger side, one line goes off through the firewall (HVAC controls, presumably), and the other went up along the passenger fender towards the battery tray and then ... nothing. It just dead-ended at a broken-off plastic line. Capped it with a tiny bit of hose with a screw in it for right now. I need to attack the vacuum system hardcore someday with a Chilton manual and a buttload of various-sized rubber hoses;
4. Scraped the old nasty tint off the rear window and driver's side quarter window. Either this was some super-cheap one-layer crap with really weak adhesive or they had already peeled off the other layer beforehand, because this stuff came off sooooooo easy - no scrubbing, no PITA scraping, it just peeled off with a bit of persuasion now and then with a razorblade;
5. Cleaned off some mysterious oil-like substance that had been sprayed upon all the inside window surfaces. Yayyyyyy, I can see again!
6. Fixed the wobbly rearview mirror ... although I need to replace it, because the inside silver/chrome lining is all tarnished and spotty and kinda difficult to see through;
7. Fiddled with the rear license plate lights a bit. One needed to be super-glued together in two places (hard to explain), both had to be sanded off because some loser didn't mask them off when they painted the car, and I still need to poke a couple of new bulbs in there because both of 'em are burned out;
8. Put the new 195* thermostat back in. It's obviously not the problem if the car still overheats without ANY thermostat in place. Topped off the coolant and burped out all the air bubbles;
9. Vacuumed the car AGAIN (damned window tint pieces);
10. Re-torqued some of the valve cover bolts that had worked themselves loose. Might need gaskets later, but for right now, this should help because almost ALL of the valve cover bolts were VERY loose - not even finger-tight.
It starts up a little easier since I capped that vacuum line, but it still idles low (need to adjust the curb idle), and it runs a little funky because I'm sure I don't have the vacuum lines 100% secured and routed correctly. These friggin' speed density cars are sooooooo sensitive to vacuum leaks, and of course there's a hundred damned hoses to chase down under there...
The car still doesn't smoke or anything, exhaust doesn't smell weird. I'm pretty confident that once I get this cooling issue ironed out - I'm still leaning towards it needing a new radiator - and I make sure the vacuum lines are all properly routed and not leaking, this car should zip right through smog inspection and be fine. Then I can start tearing into stuff and making more headaches for myself...
