The thermostat sets the
MINIMUM
coolant temperature.
When the coolant temp
FALLS
below the rated opening, the stat closes, in an effort to keep the engine warm. The stat has absolutely NO effect if the coolant temp is hotter than it's rated set point. It just sits there WIDE-OPEN, waiting to see the coolant temp fall to less than it's set-point, when it jumps back into action.
However, if the stat fails it's job and gets stuck closed/or near-closed ; now you got trouble.
The pressure-type rad cap does nothing but raise the boiling point of the coolant, so the auto-manufacturer can run a smaller rad, with an inferior coolant,in a smaller liquid capacity system; it's all about saving a couple of bucks to them.
That cap should never blow-off, unless the pressure rises past the blow-point, in response to the liquid being too hot; which, if it happens at speed, points directly at an efficiency failure (except as already noted, that you over-filled it.)
If it happens only at idle or low-speed operation, this then includes; the fan, the shroud, the water-pump speed, and belt slippage; and above all;
Air-flow thru the fins and water-flow thru the core.
So your first defense against overheating is to check to make sure the rad freely passes air thru the fins, and water freely runs out the bottom hose very near to the same rate as you can pour it in the top.If you have an A/C condenser in front of the rad, it too must freely pass air thru it.
After that; is to check the operation of the stat, or just replace it; they used to be like 3 bucks...... so in a shop setting we just threw them away.For diagnostics, you can take the stat out and break the guts off it, then re-install the disc. If it still overheats, then you have other issues.
The next thing, after that if you have one,comes the fan-clutch the hotter the reservoir gets, the more difficult it should be to rotate the fan. Some are designed to fail closed; so when the reservoir is cold, it can also be difficult to rotate. Between NOT cold and NOT hot, the fan will vary it's resistance between slipping a lot, to not slipping at all.
All this assumes your engine is NOT blowing compression into the water-jacket, as already mentioned. A compression test will reveal a problem mostly in the top-most inch of piston travel. It will not always find a problem lower than that.
But, if you gut the stat and put just the disc back in,then fire that beast up, if there exists a pressure dump into the cooling system, yur gonna see the bubbles in the top of the rad, together with stinky steam, almost right away. Resist the urge to light the steam on fire.
That's all I got.
Oh wait; you said 89. You still got the lean-burn?
If yes, then prove the diaphragm in the V-can is still functional. If it fails, your engine will run FULL-TIME retarded timing. It will be gutless, suck gas bad, and overheat; and no amount of cooling system diddling will cure what ails it.
So after proving it holds a vacuum; you still gotta make sure the thing advances the timing.
If you don't have a LB, then map out your timings and make sure the engine is seeing adequate Part-Throttle timing. And make sure TDC on the balancer is actually TDC on the piston.
Adequate timing will vary with the application, but I suggest 10* at idle, and a minimum of 32* @3400@WOT, less if it detonates at "normal" engine temps of 180 to 200. And in between the Vcan should be pulling full timing ASAP, usually 10 to 15 degrees by 1800rpm at small throttle settings. This is my opinion. If the engine accepts more without detonating; give her what she wants. If dshe will not accept these "adequate" numbers, you got some other issues. Start with cleaning out the heat-crossover under the carb, and proving the heat-riser, and EGR system, and restore the heated air intake system.
Your carb is calibrated to run air at a certain temperature, and it ain't more than 200 of underhood air, nor ambient . IIRC it is calibrated for a constant air-temp of less than 100*F, 80/85* rings a bell.