From my research on this question last winter, this is what I think is the situation. The 551 uses the same gear box ratio as the 450 (which is the one designed for your tractor). The problem is that the 551 has a larger input pulley. Plus the two idler pulleys are not parallel to each other, but instead spread out as you move away from the tractor. This may mean that you can not use a smaller input pulley. It does not end there. The input shaft on the gear box is different then the one on the 450, so a pulley swap may me tough to do. Hope this helps.
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