[Jeza]

Once you get your air supply in and out sorted you can improve the effeciency of your radiator by turning it into a double pass radiator.

This uses baffles to force the water throught the radiator twice, ie through half of the radiator first (eg the top then through the other half (eg the bottom).

You may be able to use a smaller opening (like SteveC) or a smaller radiator if your effeciency is good.