Does it burn oil? If not it sounds like its pig rich. I'd check the ignition timing first and then start pulling fuel out by leaning the jets.

I know my 360 (with magnum heads) has the stock plugs for a 96 ram and a holley 600 that came with the engine when I bought it and man the thing is awesome. Cleanest exhaust i've ever seen on a muscle car and it runs great too.