What would I-76 be in Colorado (at the time I-80S)? 
Possibly I-72, if the renumbering of I-80S occurred before Illinois got the number for the Springfield - Chambana corridor. If the I-80S renumbering occurred after the creation of I-72 in Illinois, I don't know for sure, but would guess either a western I-72 or I-78 - I'd lean towards the latter since the two I-78's would be separated by a much greater distance than the two I-72's. Of course, it could have also been redesignated as a duplicate of I-74, but since I-74 was longer than either I-72 or I-78 (and to me seems like a more important route than I-72) I'd hope not.