I eyeballed a couple of other boats at our yard on the weekend when we hauled out for a scrub and paint and from what I saw the Harken type block and tackle system would work more than adequately on the Raven. Initial attachment point would be about 500mm above the bimini top. With a four to one system that should be manageable.
Your post does however make me think that the Johnson system might well be problematic as regards height of attachment point with a twin backstay.
“Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life.” Terry Pratchett