It's an air pocket. No big deal. Here's what I do when adding coolant after a mod - once the radiator is filled, I unscrew the ECT sensor - the highest point of coolant on the motor, then use a funnel to add coolant, slowly right though the ECT opening. When it's full, it'll seep out. Then put the ECT back in, Fill the radiator about 80%, start the car with the radiator cap off and the heater on high. Wait about 4-5 minutes until the thermosta opens - the coolnat will suddenly drop - and then top off the radiator and put the cap on. Be sure the overflow has plenty of coolant in it. Then stop the engine. Let it cool, and pop the radiator cap and topp it off again. Should be good to go - you may have to drive it and do a couple of hard stops or quick right/left turns to work all the air out. But if you filled the overflow enough, the air'll be burped out and the coolant will be sucked back in when it cools.
Don