US 84 to Oregon would have prevented I-80N from being renumbered to I-84 
. And the Interstates were already under construction by the time of that letter (1959).
Ha!
If I were a betting man, I'd say that I-80N would've become I-82 and I-82 something else--maybe I-86 (as this was during the time of the suffixed Interstates)?
US-84 to Oregon is definitely one of the more out-there proposals. Crescent Junction was mentioned in those documents, so I can't help but try to connect the dots for US-84 between there and Bliss; it'd probably go something like this:
US-6 Overlap to Spanish Fork
US-91 Overlap to Brigham City
US-30S Overlap to US-30N/30S split
US-30 Overlap to Bliss
The timing may have worked out as well, given that 91 and 30S were on their way out within 15-20 years, with US-84 possibly supplanting 91 to Brigham City and 30S back to 30 once Interstates 15 and 80N were being built. Of course, US-84 would've been overlapping 80N once it was completed.
Also another tidbit: IIRC part of former US-91, after it was decommissioned south of Brigham City in 1974 and before Utah's 1977 renumbering, was numbered UT-84. (Correct me if I'm wrong, though.)
