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Look at the "single clutch assembly" parts pages for the 841 (which your 4 speed 2000 is basically the same as) and the 851, and for your 2000. The drawing is the same and the part descriptions are the same on all of those pages, so what you are saying is basically true, that the 2000 used some of the the same clutches as the 851 (and some the same as the 841 for that matter).
But read the descriptions closely, especially the multiple lines for part #7 in the drawing, which is the clutch disc itself. Each line is a different part number for a different clutch disc, and they all specifically say either 4 speed or 5 speed in the description. So you cannot use a 4 speed clutch with a 5 speed transmission, and vice-versa. That is what he meant when he said that you need to have the flywheel and clutch from a x51 to match the transmission from the 851, although technically it could be a clutch from a 5 speed 2000 as well.
Here are a few of the descriptions as examples (bold added for emphasis):
Disc Assy., Clutch, Single 9", Regular 4-Speed Transmission, W/8N7563, Except Offset, Heavy Duty Industrial
Disc Assy., Clutch, Single 9", Heavy Duty 4-Speed Transmission, W/09A7563, Except Offset, Heavy Duty Industrialw/springs
Disc Assy., Clutch, Single 10", 10 Splines, Regular 4-Speed, Before S/N 14257, W/NDA7563A, 2000 All Purpose Only
Disc Assy., Clutch, Single 10", 15 Splines, Regular 5-Speed
Disc Assy., Clutch, Single 10", 10 Splines, Heavy Duty 4-Speed Transmission, Before S/N 14257, W/311485 & B9NN7563A, Except 501, Heavy Duty Industrial
Disc Assy., Clutch, Single 10", 15 Splines, 5-Speed Transmission, Replaces C0NN7550C