I very nearly considered running this with B/X D&D, now that it's in print again via DriveThruRPG. I've recently been editing a game (for somebody else) that is basically fantasy post-apoc with a unique premise (I won't mention it here). It got my brain firing on all cylinders and was the initial impetus for committing to this campaign.
After some research (including polling some Facebook groups), I decided to use Gamma World 4e as it was deemed to be the most accessible, least error-riddled, and most complete in one volume version of the game (all selling points for me). I decided on a more traditional approach to the apocalypse (the bombs!), which is where the Morrow Project stuff comes in.
I will probably set up the PCs as the equivalent of a Morrow Project recon or MARS team, waking up in a bolt hole near my current residence (Colorado Springs, which has an alarming number of Civil Defense fallout shelters under the city center, in addition to the nearby Cheyenne Mountain complex). Anyway, that's my current idea (subject to change).
[Edit: Alternately, I might place them in central Kansas (near the town that I grew up in) which was home to roughly a dozen Titan II missile silos and other Cold War facilities until the early 80s.]
The Darwin's World gazetteer lays out a pretty good overview of North America that should mashup with the Gamma World setting information (I mentioned the different Gamma World factions, which I think are great and should be easy to drop into a homebrew). I need to read that in its entirely, though.
