Most of the overheat issues are among a few things:

1. Clogged or partially clogged radiator.
2. Broken heater hose under the dash or behind the intake manifold, or the water pump to bypass pipe hose...not normally changed like radiator hoses.
3. Water pump impellers eroded away, and thus no flow.
4. Closed thermostat.
5. In some cases, folks change to the Starquest engine, and then turbo boost can affect the gaskets. Blew the headgasket on my MM this way, too many rooster tails in the pasture.
6. Initial headbolt torque from Mitsubishi was too low. Later on, they modified the torque requirements.

When you're doing a head gasket, it is typical to replace the hoses mentioned, rod out the radiator, change the water pump, etc.

I love the earliest of the Monteros like yours, little quirky things that weren't continued after the 85 model year. I searched long and often for the split rear bumpers to install on my Raider. Someone sent me a pair out of the blue.

Underneath the front hub caps, I'm curious if you have locked manual hubs (most likely), or auto hubs?

Cheers!