Welcome to the forum!
Several things. One, a 551 is designed for a much newer tractor than an 800 and uses a 5/8" belt. All the attachments for the 800 are for a 3/8" belt. The tractors that the 551 is designed for have engines that sit further forward.
The most important thing here is that the 551 takes a minimum of about 16 hp to run it. There is NO WAY the 8 hp engine in the 800 can run a 551. Your best bet here would be to either sell the 551 and get a QA36A (the correct thrower for the 800) and a creeper drive if the tractor doesn't already have one (necessary for a snowthrower) or get a newer tractor that the 551 will fit.
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