There's a couple of problems -- as written you are going to replace the first (up to) 40 characters of the Clipboard text with the literal ^.{,40} and then the date in numeric format, keeping the rest of the text as-is.
To do what you want with a search-and-replace you need to match the entire Clipboard contents, "capture" the first 40 characters, and use that "capture group" in your replacement:
(Either $1 or \1 can be used for "the first capture group".)
That will still break if your Clipboard contains any end-of-line characters because . does match those by default. So a better pattern would be to turn on ". matches EOL" with (?s), match up to 40 non-new line characters as a capture group, then match everything else:
Though this is one of those case where "more efficient" doesn't mean "faster to execute", because you are adding a millisecond with the extra action.
Use the regex search version if the contents of the Clipboard might ever have a Return character in those first 40 -- the regex . won't match that so you will never get a (problematic) Return in your new filename, but you could if you use the substring method.
With this test macro most of the times only the first step is executed and the “file name area” is not selected, as it should be with normal ⌥⇧⌘ S (for TextEdit).
After a reboot, I tried 4 more times and it worked twice and twice it didn’t, so that's bad enough.
The test macro aborts here, that's why I added a pause 1.5sec without success.