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Expansion bottle or header tank cooling system?

Yes an expansion bottle is the answer. I have the system from a Toyota Cressida on the 7. The bottle fits perfectly between the stock radiator & the inner guard. Even the radiator cap fits. I never have to add coolant between changes.

However for the first fill when empty the local radiator shop has a good system. They drilled a 5/8" hole through the centre of a radiator cap, & brazed a short length of 1/2" copper pipe through the hole, flush at the bottom & about 2'' high out the top.

They slide a suitable funnel over the pipe, & clamp it with a couple of hose clamps. This allows coolant to be filled including the funnel, to about 6" above the top of the cooling system. 15 minutes of idling with this overfill allows all air to be cleared from any cars system they tell me.

Hasbeen

Header tank should be easy, just to it by the book:
Fill the system through the header tank's filler hole.
Take your time and regularly squeeze the bottom hose (from water pump to radiator).
Top up till the level is about half an inch beneath the filler's neck

And make sure you have the correct thermostat fitted.
It not only should have the foot, but also a small air bleed valve.
You can hear the valve ticking when you squeeze the hose mentioned.

Blockage somewhere? Or just filling it too quickly?
Overfilling the tank while there's still air in the system?

What Beans said. But you have to do it slowly. Alternatively, disconnect pipe at expansion tank, put funnel on hose fill slowly. If you fill too fast it can trap air somewhere in the system. once it is full (to top of hose), reconnect hose to expansion tank and fill expansion tank to appropriate level. On engines with A/C, make sure the switch is to warm/hot so the coolant has access to intercooler. Those without A/C don't have that open/close valve. Never had a problem. Little bits of trapped air (air bubbles) will get expelled with use, check levels initially periodically if you don't have the low coolant light. The key is doing it slowly if everything else is right (thermostat etc)..

What Beans said. But you have to do it slowly. Alternatively, disconnect pipe at expansion tank, put funnel on hose fill slowly. If you fill too fast it can trap air somewhere in the system. once it is full (to top of hose), reconnect hose to expansion tank and fill expansion tank to appropriate level. On engines with A/C, make sure the switch is to warm/hot so the coolant has access to intercooler. Those without A/C don't have that open/close valve. Never had a problem. Little bits of trapped air (air bubbles) will get expelled with use, check levels initially periodically if you don't have the low coolant light. The key is doing it slowly if everything else is right (thermostat etc)..