Quite a lot of people seem to like 7/16 tubing on 1/2" barbs, tend not to need clips or anything to hold the tube on then and gives it a clean look. 3/8" tubing may be a bit too difficult to get over 1/2" barbs? Not sure myself, never tried. You have 1/2" barbs on your blocks and things already then?
Think your rad used 3/8" thread, best to check the spec on theyre website for info.