A 256@050 hyd cam in a 400 engine? Not surprised it runs rough, especially if it is low on compression ratio. All manner of problems I can think of with a cam of that size: PCV valve open from insufficient vacuum. Engine blubbery rich at idle because the metering rods in the carb are in the rich position. You will probably need the softest springs in the Edelbrock catalog for that cam, may be even softer. The carb will also need the idle circuit to be recalibrated. Plugs are probably loading up because of the rich idle, one or two may not be firing causing the roughness that you are describing. If they are covered in black soot, they are probably misfiring. You want NGK plugs in a 5 heat range, projected nose core. To get the car to idle without overly loading up, remove the carb metering piston springs & drop the piston/rods assys all the way into the holes. Block the PCV hose. With clean spark plugs, you should at least get a stable enough idle to check for a cylinder out, but the engine is probably ok, just needs the above to be sorted.
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