The stock intake (dual plane) has two plenums and each feeds the outside two plugs on one bank of the engine and the inside two on the other bank. If you can remember which ones were where like Vikki said or check the new ones for colour, you may be able to isolate which side of the carb may be dumping fuel like Tom said.
The left side of the carb feeds into the left H-shaped plenum and supplies cylinders #1 and #7 on the left bank and #4 and #6 on the right bank. The right side of the carb feeds cylinders #2 and #8 on the right bank and #3 and #5 on the left bank. Since #1, 7, 2 and 8 are the ones (corners only) that are black I'd be more inclined to suspect that oil is getting past the guides during "spirited" driving (sloshing to the front and rear in the valve cover) and fouling the plugs. They may have gotten the guides and/or seals wrong during the rebuild.
The carb could be dumping extra fuel and doing the same to the plugs but it sounds like the engine is running pretty good now. A check of the new plugs might be very handy to see what is happening now before going deeper.
