I don't think junk in the block is the problem; not after 30 years. What happens is casting sand gets stuck in a corner in the back of the block and often promotes corrosion of the core plug, then coolant leaks from the transmission bell housing. When that plug is replaced, there is a handful of sand, (mud) that can be pulled out first. Even that doesn't cause overheating.
It sounds like the radiator is doing its job with the coolant in it, but based on your description of getting hotter as soon as you accelerate, it almost suggests there is insufficient coolant flow. When you accelerate, more hot coolant flows to the temperature sending unit causing the gauge to rise. Check the fan to see if the belt is loose or slipping. A V-belt can be tight but still slip if it is worn. The sides are supposed to wedge into the sides of the pulley. When the belt wears enough, it will ride on the skinnier bottom and the sides no longer grab the sides of the pulley. That lets it slip regardless how tight the belt is. The same thing can happen if the belt is too skinny to start with. With low flow, you might also find the temperature of the air from the heater is lower than normal when the fan is switched to one of the highest speeds but that can be hard to judge in the summer.
Have you needed to add coolant regularly lately? If a head gasket is leaking, combustion gases will be forced into the cooling system and you would see bubbles in the reservoir, including before the engine warms up. Coolant would also sneak back into the combustion chamber so the level would go down in the reservoir. If that isn't happening, I would find it awfully hard to believe a head gasket is leaking. That just was not a well-known problem in the '70s when engines had cast iron heads and block.
If you DO see bubbles in the reservoir, that could allow the thermostat to close and lead to overheating. Thermostats only respond to hot liquid, not hot air. Air pockets in the Chrysler engines was not a problem in the '70s because the air would just sail right past the thermostat and collect in the upper radiator tank. I could see if there was enough air leaking into the coolant that the thermostat might start to close but I still would need to be convinced. If you ran the engine without the thermostat and the overheating stops, I'd still suspect the thermostat first before a head gasket. If you try removing the thermostat and it overheats even faster, try squeezing the upper hose to restrict coolant flow a little. It is very possible to CAUSE an overheating condition by removing the thermostat where there was no problem previously because without it, hot coolant will go through the radiator way too quickly and won't have time to give up its heat to the metal tubes. The clue is you'll find the lower hose is much hotter than you just found it to be.
As for flushing the cooling system, all I ever did at the dealership was to remove one heater hose from the engine and the lower radiator hose, then run water through the heater hose and port from a garden hose. I've heard some stories about plugged heater cores causing overheating because the return coolant, (which is still pretty hot), is what helped the thermostat to open. I'm not sure I believe that story but it would be interesting to see what kind of flow you get through the heater core and whether it improves after flushing. I don't think you're going to see anything come out from the block that hasn't already broken loose years ago.
Wednesday, June 1st, 2011 AT 3:07 AM