I found this graph on Wikipedia illustrating how fast the buildup was: http://en.m.wikipedia.org/wiki/Historical_nuclear_weapons_stockpiles_and_nuclear_tests_by_country. The sheer amount of manpower and material that goes into making that many warheads is astounding, and this was pre-automation. The number of people involved must have been enormous, so where did they end up when production stopped? Where did all of the factories and equipment go?
That's a really good question. The cited references on the Pantex and Rocky Flats wiki entries would be good places to look for folklore if not data. These sites were where the production was done.
I would not say this was pre-automation — they had plenty of automation back then. It was pre-robots, sure. But there is a lot more to automation than robots. (And, in fact, a lot of early remote automation technology was invented for the purpose of handling radioactive materials, as an aside.)
How'd they do it? They built a lot of facilities, hired lots of people, spent a lot of money — at least $5 trillion in constant 1996 dollars. (Actually building the warheads themselves was only 7% of the total cost.)
To make nuclear weapons you need fissile material — enriched uranium or plutonium. During the Manhattan Project the US government built 3 early industrial-sized plutonium-production reactors in Hanford, Washington. They also built several uranium enriching plants in Oak Ridge, Tennessee. In the early Cold War they built more reactors at Hanford, as well as at a new site called the Savannah River plant, in Georgia. They also worked to make new reactor designs that produced more material, and develop new extraction techniques that improved their plutonium yields. And along with building more plants at Oak Ridge, they also built another uranium enrichment site in Kentucky called the Paducah plant. They also built many other plants for component fabrication, high-explosives production, electronics, fissile-material shaping, and some additional enrichment sites. And they also had sites for weapons assembly and disassembly.
Parallel to all of this they also found ways to reduce the amount of fissile material in each weapon, allowing them to build some 20 warheads with the same amount of uranium that was used in the Hiroshima bomb.
As for the number of people involved, it varied over time. They did eventually close down plants and people got laid off. I don't think it's any different than any other kind of defense industry that ebbs and flows depending on technological changes, domestic and international politics, and economic factors.
As for the factories, almost all of them have been shut down. The Wikipedia section on the US nuclear weapons production complex gives a nice overview of the major sites that were created and their status today. The ones that had a lot of nuclear waste or fissile material contamination have had to go through significant remediation and some are still among the most radiologically polluted sites in the world.