Which is the closest major UCAS city to Denver, circa before Ghostwalker?

Depends on what you mean by "major"...I'll take it to mean cities that get marked on SR maps of North America.

Here are the closest in order

Witchita (approx 439 miles)
Omaha (approx 482 miles)
Topeka (approx 491 miles)
Sioux Falls (approx 492 miles)
Bismark (approx 528 miles)
Kansas City (approx 557 miles)

That will do quite nicely, thank you. My first guess would have been Kansas City, but my American geography is quite horrible.

Well, currently, the biggest and closest city to Denver is Colorado Springs (2nd largest city in Colorado) Its a fairly large city, though not super urbanized like Denver. Fort Collins is a bit smaller still ( my neck of the woods ) but within 50 miles.

If you want something just as big as Denver though, you may have to hit KC or Las Vegas, or even Salt Lake City.

For some reason, Las Vegas seems closer than Wichita, KC or Omaha... maybe Kansas and Nebraska just seem so damn boring when driving through them..

I won't tell my wife you said that. She's from Omaha and can be touchy about criticism of it.

I would vote for Omaha for three reasons:

1. Lots of major highways converge on Omaha, and unlike Europe where rail is king, highways are the most important mode of conveyance.

2. Omaha is currently a major telecommunications hub today, and there is no reason it won't be in 60 years.

3. Omaha and its annexed cities are over 1 million in population.

That is my 2

If you want a SERIOUS big city, you're talking St. Louis on the UCAS/CAS border.