slant is running hot

rather than throw parts at it id like some ideas. i really dont want to buy a new rad for it, but i think that is the culprit. help me rule out anything else before dropping the cash. car will sit and idle maybe 20 min and start getting hot, it will just keep climbing and eventually boil over. i would say you could drive it the same amount of time. i pulled the center of the thermostat out as i thought it stuck shut. it didnt seem to do much. it was a 195 t stat. i put a new 160 in. seems to run longer. i flushed the cooling system out several times. an awful lot of rusty water coming out. i ran a gallon of vinegar through it hearing that is supposed to clean out the rust. im not getting good heat inside the car even when the temp is up there. i ran the garden hose through the heater core too, maybe that is unrelated. withthe cap off the rad you could see the water wasnt flowing that well down the tubes you could see through the opening. i ran a wire through them and they seemed to drain better. but that is only a handful of tubes. i got a price o na new rad for 270, more than i want to spend. this car was supposed to be a big block swap. i was thinking of buying an aluminum rad and making it work, but i dont want to buy anything if the rad is not the culprit. i do not have a fan shroud, fan is about 1 1/2" from the rad. it has only been in the 40s to 60s here temp is no reason to run hot. im not getting water in the oil or steam out the exhaust but im really hoping i dont have a blown head gasket in a way that a cylinder is blowing into the coolant?