What thermostat do you have in the eng ?
If the rad or block are gummed up a bit the truck will heat up when pushed , check if water flows from the drain bung on the lower left side of the block .
Standard gears ? then cruise at 45mph . Should last forever then
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Low comp on one cyl could easily be a valve burnt . A squirt of oil down the bore , and another comp test will help eliminate the rings as the fault.
