I was hoping someone here had come up with a solution better than what I found on my Comet. Not getting any response, I did some more searching.
The most annoying part of the search is you can't find it with "return"; instead, it's "retracting".
So...I found a drawing in the 1964 Falcon/Comet service manual that does, indeed, show a loooong spring running from the "carburetor bellcrank" to the end of the intake manifold - but no part number. The distance is approximately 11". Then I found, on another site, the equivalent drawing which does give a part number, 9737. BUT I have been informed that this is a generic Ford part number for throttle return springs. Be very careful if you search for "9737" because it may be for a completely different engine! I have just gotten bit by this; I bought a 9737 on eBay and it won't work at all.
1964 Comet Service Manual drawing from page 328:
Unknown year or model with generic part number:
