The Crossover UniverseTM is a companion blog to the books Crossovers: A Secret Chronology of the World Volumes 1-2 by Win Scott Eckert, and the forthcoming Crossovers Expanded Volumes 1-2 by Sean Levin. Material excerpted from Crossovers Volumes 1 & 2 is © copyright 2010-2014 by Win Scott Eckert. All rights reserved. Material excerpted from Crossovers Expanded Volumes 1 & 2 is © copyright 2014-present by Sean Levin. All rights reserved.
Was this one in the original Crossovers or am I thinking of another Wellman novel?
Nope, I just checked, and it's not listed in the originals.
I don't have a copy of Crossovers with me, it maybe Wellman's What Dreams May Come, I was thinking about. (Not to be confused with Richard Matheson's novel of the same name.)