The problem is that the passage in the head is above the radiator and that's where the bubble naturally migrates to.
Think about "burping" your cooling system (yes, this gets discussed on a pretty regular basis.) There are several methods.
One is to jack up the rear end so that the engine now sits nose down, rather than nose-high. Remove the temp sender/sensor from the rear of the head (single wire hookup, driver's side by the valve cover) and fill there.
I played games with the head on my XJ last fall, replaced a head gasket the day before I left for Moab. I watched the temp gauge bounce around driving across IL, and half way across IA before the bubble in the head finally got pushed through the thermostat and into the radiator where it could escape through the cap into the overflow. Yes, it took several hours of hard running.