[400TT]

In my experience the best solution is to fit an in-line filler neck and an over-flow tank.

In-Line Filler Necks and Over-Flow tanks are available off the shelf. You can even use the plastic over-flow tanks from cheap auto stores. The only problem with plastic ones is that they discolour and look terrible after a while. Even when new they are not the most attractive thing.

Snake Bite sell both anodized CNC machined T6061-T6 alloy In-Line Filler Necks and polished stainless Over-Flow tanks.

You want to fit your in-line filler neck to the top radiator hose, ideally so that it is the highest part of your coolant system. If it isn't the highest part, then it will be more difficult to get all the air out of the system. You can fit a small bleeder to the top of your radiator if it's the highest part to bleed air out of the system when filling with coolant. Or if the top hose is the highest part, just push it down to burp all the air out. It can also help to get all the air out, by filling the car with coolant while sitting the front up on ramps/axle stands. You can also run the car with filler cap off to bleed off some extra air, and give it a little rev to squeeze a little more fluid in and quickly put the cap on.

Our first Cobra had an in-line filler cap that was below the top of the radiator. We would just fill the coolant as high as we could then run the engine, the engine would eventually bleed out the extra air into the over-flow tank. When the car was cool, we would keep topping it up until all the air was removed.

Once all the air is removed and you engine is up to temperature, you should see the over-flow tank level rise as the engine bleeds excess coolant into the over-flow tank. When the engine is cool, the over-flow tank level should drop as the coolant is returned to the cooling system. If you have air in your system, the over-flow system normally does not work.