Block-off plate, as I learned on this board, is a notorious leak spot. The rounded ends of the plate (top or bottom) are usually JUST long enough to interfere with a casting edge, so the plate doesn't sit flat on the gasket.
I had a passenger side valve cover leak that drove me nuts. Finally found that one of the bolt holes (in the rebuilt heads I'd bought) had some crap in the bottom and although torqued to factory spec, was not holding the clamp tight against the cover. Cleaned and chased the hole - no more leak!
Well, at that spot, anyway. The IDI even beats my two BMW M62 V8's for "leakiest sumbitch ever made" as someone once posted on a BMW forum.