I give him also Arizona. @Kingpoleon: The EV. in your scenario is also 491-47.
President Stuart Symington/Senator Hubert Humphrey: 491 EV. (61.3%)
Senator Barry Goldwater/Representative William E. Miller: 47 EV. (38.0%)
Yeah, Symington has fairly broad appeal, and he could hardly seem like a worse person than Johnson.
He's also a Generic D without the same clout, and likely would not attack Goldwater nearly as relentlessly.
It would take merciless attacks to win as a D in Utah, Idaho, or Virginia period. The only reason the Upper South would hold is because he's of Missouri. Only JFK or LBJ could do those things.