I am concerned about this project because of some of the language that has been used by the state and local governments when discussing it. The "boulevard" and "community grid" have a level of touchy-feelyness that I don't feel would exist if the department were leaning toward replacing the viaduct. Just my opinion, but it seems like an awful lot of effort is going into discussion and visualizations for potential hipster gentrification of where 81 currently stands.
Also of concern to me is the creation of I-781 in the North Country. I don't know enough about it, but it seems like it is leaving potential for a 581 and 681 in Syracuse if the viaduct is eliminated.
I firmly believe that I-781 was chosen for the North Country because it's in NYSDOT Region 7, much like NY 840 connects Route 8 to CR 40 or NY 747 leads to an airport. I-781 was originally to be numbered NY 781.
NYSDOT can be creative with numbers once in a great while. 
It is also worth noting that, while not in current use, NY 181 and NY 381 are former designations. 181 was in Jefferson County not far from current I-781, while 381 was in Rensselaer County.
Most of the designations between 500-899 are either SR extensions of Interstates or a number containing the route(s) it bypasses, provides an alternate for, or connects. These combinations are:
-NY 531 (freeway bypass of NY 31)
-NY 598 (connects NY 5 and NY 298)
-NY 631 (alternate of NY 31)
-NY 695 (connects I-690 and NY 5)
-NY 812 (spur of NY 12)
-NY 840 (connects NY 8 with Oneida CR 40)
Two additional ones that are proposed are NY 546 (connects NY 5 and NY 146) and NY 646 (connects NY 146 and NY 156).
Of the others, you have NY 747 (obvious reference to the airport), NY 635, NY 825 (no idea where those come from) and NY/I-781 (possibly due to location). It's also very possible that I-781 was chosen to avoid confusion with I-181 in Tennessee and I-381/581 in Virginia.