with your issues when did the engine overheat exactly ?
after this overheating what was the compression ?
you replaced this O2 sensor after this internal coolant leak ?
what is the vacuum at idle ? @ RPM ?
some Items to replace with the age of this engine : EGR , radiator if its full of scale and or the cooling external fins are corroded , fan clutch.
the 3 core radiator is required when towing or in a operation of hauling with these vehicles replace radiator cap 15psi.
coolant replacement every 2 years is required. distilled water is what I use with engine coolants ...
having owned this engine for 17yrs 380,ooomi I found that if properly maintained these engines are very durable.