Did your old 400 ever idle that you know of, prior to the striped spark plug holes? Just trying to guess whether the problem is with your old stuff or the newer block. I am a lot more of a two-cylinder guy than a four guy; but from what you have said and considering the swapping situation, I would double and triple check the belt alignment, before getting too deep into the exotics like Mercury reed valves. The other easy thing is to make sure that the two throttle plates are opening at EXACTLY the same time.
Long live American manufacturing!
