If you order a 120 link chain, you may not be able to go back to your 50 tooth rear sprocket. If you order a 122 link, it may work with both your 45 and 50 tooth rear sprockets. I know that my 122 link chain worked for both my 47 and 50 tooth rear sprockets, so you won't really know unless you try. If you end up needing to shorten the 122 down to 120 links, that is not big deal. An X-ring chain should stretch very little after initial installation and the first few miles.
Hope this helps,
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