From above -
If you have to have a 38-40 start with a 45 colt and most of the headaches are gone.
I have tried this route. It may work. Though a spacer may have to be made for sizing, or to turn a little ring of brass in the head area off near the rim. It almost leaves a tiny belt, that from my experience made it so the brass would not chamber.
The thickness of brass (I suppose depending on brand) in the neck is what I was looking for as I had a chamber (38 WCF) a bit big in the neck area. Being a few thousandths thicker I wanted to try as I was experiencing terrible gas cutting. PSB added to the load cured it with stock Starline 38 WCF brass.