Slant Six overheat at cruise speed, especially going uphill

It's a flow problem for sure as everyone else noted.
Due to lack of available parts in your area, you can try this without having to buy anything (Yet anyway)
Disconnect your radiator hoses (The top one pull from the engine and the bottom one from the radiator)

Pull the thermostat housing and point the lower hose at the ground as not to blow crap all over your engine compartment.
Put a water hose AND and compressed air nozzle both into the manifold where the thermostat went and wrap them both with a towel or rag to make as good of a seal as possible.
Turn the water on as high as it can and about every 3-5 seconds hit it with the air while holding the towel down tight on the intake.
The water will get blasted through the engine block backwards from the normal flow direction and you might be suprized how much crap comes out of the block.

Next do the radiator the exact same way at the bottom hose flange and blow it out the top hose.
This will "Backflush" the radiator.

No cost at all, just some of your time and may be well worth it.