I'm in the middle of converting a shotgun to 32-40WCF using 8mm barrels right now, for most of the reasons you listed. 32-40 is a respectable medium cartridge that with a heavy bullet can perform fairly well on most game in North America, it is a low pressure cartridge that happens to fit the .323 barrels. And being in the black powder category and limited to 40grs it is a lot safer for shotgun conversion. The only thing it lacks in your criteria is the velocity.
I'm not trying to discourage you, if you want to wildcat a cartridge more power to you, one of my favorite cartridges was a wildcat up until recently(338-06), but why reinvent the wheel when their are cartridges available that will almost do what you are asking without the expense and the headache?

C