I worked on the powertrain for those back in 2000.
The chassis is a GM truck, Duramax diesel and Allison transmission.
The body is of course completely custom built. Never saw it in person, we had one guy with security clearance who was cleared to visit the build site in Detroit. As with a lot of these deals it was hidden in plain sight, an unmarked building in an industrial park.
I saw it when in DC a few years ago......his execu-parade stopped all traffic in all directions for 30 min at least.
I was so looking for rotten tomatoes to throw when I saw it coming....but then, as with the helicopters, there is a line of about 5 or 6 “Beasts” that all look identical, and you don’t know actually which one he is in...........