Vintage Cars: MGB Radiator refill, degree thermostat, expansion tank

Question
I just installed a new radiator in my 1979 MGB. I have to put in antifreeze but it only has a expansion tank and no cap on the radiator itself. Do I just put antifreeze in expansion tank and will it fill engine and heater core without starting car?

Answer
There is a plug on top of the thermostat housing. It will be difficult to unscrew because
of time and corrosion. You undo it counterclockwise. Once the plug is open, you may fill
from the expansion tank. However, please put in a new 180 degree thermostat at this time.
That is a 180 not a 195 thermostat. Measure the diameter of the old thermostat and find
a new thermostat with the same diameter. I also prefer to put a 1/8th hole in the thermostat
as an additional air bleed.