Sounds like you have a problem that caused it to overheat, and that the head gaskets are toast becuase of it.
If you are seeing coolant forced into to reservour, and bubbles there or the radiator, that is a good indication.
You can do a compression check to see if a cylinder is low.
You can check your plugs and see if you see one that is clean, and 5 others that are not.
You can get the radiator pressure tested to see if it is getting combustion pressure
You can get the radiator checked for combustion chamber gases (a sniffer)
I personally would replace the thermostat and see if that fixes it, make sure that your fan is working right and not causing it to overheat, and then check for the head gasket problem.
My head gasket problem occured on my 89 Cougar because the little tube to the coolant bottle popped off, causing it to dump the fluid on the ground and not suck any back in when it cooled off, running it low on fluid and eventually overheating with the wife driving it away from home in 120 degree heat.
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