With the work you've done recently taking the cooling system apart, it might be that you still have air in the system. I have to jack up the front of a car that I have to get it to bleed all the air out, plus I installed a flushing tee in one of the heater hoses to burp the air at the tee as well.
The other possibility is that if you were having cooling problems, you may have overheated the engine and you now have a warped head or just a blown head gasket that is allowing cylinder pressure into the cooling system and pushing coolant out of the radiator, into the overflow and overflowing the overflow as well...
You can check by taking off the rad cap when it is cool, start up the engine, and once the coolant is up to temp and flowing, look for bubbles at the water neck. This may not work if the engine has to be under load to build enough pressure to overcome the head gasket and leak.
I have used a block tester kit from Napa that chemically checks for exhaust gases in the coolant, and it its pretty accurate. I think I spent $50. It did find a leaking head gasket when the bubble check said everything was OK.