For context, I am developing an alternate history that features a far more ambitious Voyager program than the one undertaken in real life. However, despite the increase in NASA funding provided in the alternate history, the amount of new hardware and mission expertise required for the program is most likely too much to make a 1977 launch feasible. Given the rate of projected hardware and software development and the state of space exploration at the time in the alternate history, 1981 seems like a much more reasonable launch year. The obvious problem is that I do not know of any launch window around this time that could be used to achieve a "Grand Tour" trajectory, and especially do not know what later trajectory adjustments would be needed to perform a Jovian gravity assist to any of the other outer planets. Any information on potential launch windows and trajectories for a 1981 mission would be greatly appreciated, especially if it came from space exploration concepts made at the time.
NOTE: A "Grand Tour" trajectory is defined for the purposes of this question as any trajectory in which a Jupiter gravity assist can be used (with little added energy) to direct a probe to any of the 3 other outer planets (Saturn, Uranus, or Neptune) and/or Pluto within a timeframe from launch of less than ~15 years. Examples of such trajectories include the proposed 7/22/1976 to 8/6/1976 Jupiter-Saturn-Pluto window, the Voyager 2 Jupiter-Saturn-Uranus-Neptune window, the proposed 8/26/1977 to 9/10/1977 Jupiter-Saturn-Pluto window (used for Voyager 1 until the decision to redirect the probe for a closer study of Titan, thereby forgoing the Pluto flyby), and the proposed 10/28/1979 to 11/17/1979 Jupiter-Uranus-Neptune window (all displayed in this image), as well as the (very rough) 2015-2019 Jupiter-Saturn-Neptune-KBO window for the proposed spacecraft Argo.