Yes, it will fit. I know that a 89 420 has a 722.355 and a 90 560 has a 722.350. I have no idea what the difference is but must have something to do with engine characteristics as the cars are the same weight and size. I have a 89 420 722.355 in my 84 500 and it has worked great for the last 60K miles in this car. Right now, it also has no reverse. Going into shop today. The "no reverse" seems to be a common problem on these transmissions. Some say to rebuild, and some say just fix it. The reverse clutches are in the front of the transmission and easy to get to after the trans is removed. I don't know which my trans will get (rebuild/fix) yet. It depends on what it looks like when he starts into it. We will see.

Paul

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84 500 SEL (307,xxx miles)