You mention boil over. I have only had the car for 2 months, and driven it sporadically. However, I've had several instances of it boiling out of the overflow tube
Rust in the radiator...rusted-out freeze plug…it's not unreasonable to suspect a rusted-through head gasket, too. The original head gaskets are stamped steel.
(Too late now, but when you pulled the freeze plug to replace it, you ought to have taken steps to dig out the rust muck that tends to accumulate in the block, accessible without the freeze plugs in the way...)