Overheating issues on a 383 Polara

My cousin and I were looking at a 1969 Dodge Polara with a 383-2V (converted to 4-bbl. and dual exhaust) for sale up in Fort Collins today. For $1000 and the condition it's in it's quite a bargain but the engine overheats after a half-hour or so of driving (started cold). The owner said he replaced the thermostat, hoses, water pump, and had the radiator flushed but the problem still existed. Since he had it done by a shop I'm wondering if there might be the classic "Mopar block corrosion" going on and the system is plugged up with deposits. If this is the case would there be any way to at least partially clean it out so we could drive the car on the highway for about 1 1/2 hours?