The Suburban chassis — the 3/4 ton Suburban! - was used after the Deville was down-sized and even more up-armored. Plus it has a BIG Dana 60 axle and the heavy truck front suspension. I think that was the second one I saw built.
I was told the engine still met emissions regulations back then. It was 7.4 liter (454 CID) big block Chevy. I’d guess the Beast is sporting the 560 HP Caddy supercharged LSA V8 but no one is talking!