For rank 40 ones, you need to be in the Land of Carnage (also, make sure it's your first time entering the 100th floor of that item, as it'll be randomized if you had gency-ed out from that floor before, regardless if you entered it from Holt Village or LoC).

To make things a bit easier, you'll want to Gency out on floor 99 and save.

Aside from being able to reload your save in-case your steal attempt fails, you might want to reload if your Rank 40 item doesn't have the rarity you want, since rarity matching will grant bonus stats. You can do whichever rarity you want, but most people shoot for Rarity 0.