You could partition the external (120GB) drive into more than one partition and put the Acronis image (of the 40GB drive) on the one of these.
Also create an Acronis boot CD (from within Acronis).
Then swap the drives and boot from the CD.
You should then be able to restore the image on the 120GB to the (an)other partition (as active, primary) on the same drive.
If you don't have partitioning software, you may be able to achieve similar effect with Acronis's Secure Zone (which is a hidden partition) if it will allow you to put it on the 120GB whislt it is still the external drive.