The coolant is red and the oil black as night... anything else to check??
I would test the coolant for exhaust gases (tests are available for rent from most box auto stores,) test the compression (in the '60's all pistons are supposed to be within 14psi, not sure what the metric is for an '80) and send an oil sample to Blackstone. There is no obvious steaming on your plugs. If that all comes back clean I would feel pretty comfortable with the truck.