Your idea was/is not without merit. I have been looking for a 38 Super slide for a Springer GI 1911 I have - a friend's Series 70 Colt 38 Super slide fit right on the frame and now that I need my own upper I be lookin', and found them at Caspian Arms. Now for the next project.......
I do not recall if Caspian produces 40 caliber slides, I didn't look that closely at the website once I found the 38 Super slides but if they do then you may have found your answer, especially if the bolt face on the 40 is the same as it would be for the 10mm - don't know about the ejection port, although they may all be the same size. Just a thought.