Hate to say it, but I think you should just get a JCM900. If that's the sound you want, nothing will be as pure as the real deal. And they aren't very expensive and are widely available used.
Just the other night I was at a show standing about 15 feet from a JCM900 w/matching standard 4x12 cab, and the guy had it dialed in just perfect. Sometimes I try for the Marshall tone out of the mkiv - class A w/EL34s, but it always ends up having that signature Mesa sound. That's not a bad thing, but its not the pure Marshall tone.