I know this is a silly suggestion but have you checked to see if the polarity on your fan is correct and that it is in fact sucking and not blowing. The AK system for cooling works really well so I am a bit puzzled as to why you are having heating issues. The stat you have is fine and as was covered in another recent thread, it is the stat that regulates the temp so putting a lower rating one in should not really solve what ever the issue is if the one in at the moment cant do it. It is a 3 core radiator in isn't it?
What engine have you got in at the moment,
for a RV8 those figures are fine for a Ford or Chevy then possibly a bit low.