I'd buy some of Adirondak Jack's "45 Cowboy Special" brass and load it like a short 45 Colt. There is lots of info located here:
http://cowboy45special.com/cowboy45brass.html
The Kirst 45 acp chamber is cut to headspace on the mouth of the cartridge and the specs allow for use of this brass as well as 45acp.
This round loaded correctly is a ballistic duplicate for the original Remington conversion round, 46 rimfire!