My understanding that trying to do a wildcat in this way often requires a new barrel.
The problem is that the neck area of the new wildcat chamber specified to be the same size as the old chamber, so reaming the new wildcat chamber doesn't cut a true neck, but may just leave scratches or not cut in the neck area at all, or you can end up with a neck that is not concentric with the new chamber. This can cause problems to eject, problems with accuracy, and even safety problems with the neck not properly releasing the bullet leading to excessive pressures. You really want the new wildcat to have a longer, larger chamber that entirely cleans up the old chamber, cutting metal away the entire length of the chamber. I believe that the 357 Bain and Davis formed by necking down the 44 Magnum they left the neck of the cartridge case very thick, not thinning the neck walls at all by neck turning or reaming so that the new cartridges outside neck diameter was large enough to clean up the old 357 chamber in the neck area.

edit: Looked up relevant measurements, 357 Magnum cartridge mouth has outside diameter of .379, 357 Bain & Davis has cartridge mouth outside diameter of .3839; these numbers from Ken Howell's "Designing and Forming Custom Cartridges".
Also the Ackley Improved cases often have this problem with the neck not being cleaned up by the new reamer, so gunsmiths would set the barrel back several threads when converting to use the Ackley Improved chamber to get a properly cut, accurate and safe chamber.

Sounds like this was something I was going to need.

B&D closed up shop a few years ago from what I read.

Also read that to do this the chamber needs to be drilled before reaming. The amount of material needed to be removed is too much and the 357 B&D reamer is for finishing, would go dull long before getting where you need to be.

I just checked both of my 357 barrels, the 44/50 bolt will not close on either of them. So there's something else to deal with.

I bet if enough of us ask MR to make us some though

Interesting. I had never heard of this wildcat, but it seems very interesting.
Like mentioned, the problem is the 44/50 bolts will not work with the 357 barrels. So it seems the DE would not be a good host for this caliber.

However a lever gun in this caliber seems like a very interesting idea.