Roamy here. NASA has to ship some pretty large components, so when barge or rail is not feasible or cost effective, they fly in the Super Guppy.
It’s officially called a 377 SG, modified from the Boeing 377, still using the cockpit, wings, tail, and main landing gear but borrowing a nose gear from the Boeing 707 and adding turboprop engines. When I saw it, it was being used to ferry parts for the International Space Station. It can haul up to 52,500 lbs.
One of these has been retired to the Pima Air and Space Museum in Tucson.
I asked one of the Super Guppy pilots how do you train for flying something that big, that one-of-a-kind. He said there is no simulator, you just get on-the-job training with an experienced pilot. He also said that previous experience flying large cargo planes helped, but that there is nothing else like it. I bet.