The U1 skivved heatsinks are preferable since the fin height is already almost optimum, I used
this one to make this block
Since these pics were taken I've modified the block to have a centeral inlet like cathars as well as other minor internal changes.
if you leave more fin area on the base and leave the base at default thickness(my block was taken down to 2mm thick) it will cool quite well even at very low flow rates.