400 BB build issues

If you came here for advice you came to the right place but only if you use the advice given above. Any cooling system will boil over when heated if not sealed off from atmosphere.
This is very simple and has already been stated above top it off with coolant then install a new radiator cap put the belt back on it and drive it while making sure it doesn't overheat
and when you shut it down if you here it boiling you have either got a head gasket or cooling system leak.