I have run across a few of those. I had a guy asking me some questions about GM axles as he had repeated rear axle issues out of a 59 he had.
If you run the vin 90% of them come up as a Hearse. Pick them up dirt cheap back in the 60's. They had extra long wheel bases. This guys father in-law built it and he had pictures of the build. Apparently in some magazine like Popular Mechanics or something they did a conversion story and spawned quite a few of those.
Sort of a shame though that it got converted. a 59 Cadillac in any model is sort of the holly grail of all Caddies.