I want to get a diesel caddy brougham, but I don't want to deal with my engine exploding or whatever happened to all those gas 350s that GM dressed up like diesels. Is dropping a 6.2 my best bet?

6.2 is a much better powerplant all around compared to the 5.7 for sure. A lot of work involved in making a 6.2 fit where the 5.7 was, the engine is considerably heavier and larger and would be a pretty tight fit.

Not to mention availability. You'd have to find an individual or salvage that had a core worth rebuilding. The 6.5L became the 6.2L replacement long ago but the price difference in a 6.5L shortblock and rebuilding a 6.2L can be $1000 or more.