The performance curve of the MCP600 pump is more to the lower end for flow, but is higher pressure than your average pump like a Eheim or Hydor. So I guess the best block for you would be the one that makes the most use of high pressure impingement, and that is effective with a low volume flow. The TDX and WW blocks really shine with high volume waterflow, whereas the the thicker base pinned blocks seem to maximize cooling with low volume flow. At least that's the pattern I see with all the review data out there.
I bet the Swifty MCW6002 block would be the best bet in your rig. The new inlet design on it encourages high velocity impingement, and its got a thick base and mucho tiny pins. But I guess we need a good reviewer to put it through its paces before you (or I for that matter) could really make a judgement there. So really I'm just guessing!
