Captain John; My son & I both have Mark VI's. lots of fun. FIOCCHI makes ammo, but pricey. You can use .455 Webley, or .450 Corto, (means "short" - same as the original .450 Adams) If yours is an older Mark, stick with Black!
To make your own cases, you can go from .45 Colt, or .45 Auto Rim. The new .45 Cowboy Special would save the shortening. Thin the rims to .035. Best to use a lathe, but i have done some with a three corner file with one side ground safe. Shorten the Colt case with a small tube cutter, and trim to .45 Acp length. The mark I case is a touch shorter than .45 Acp, but Webley chambers are tapered and will take the extra .02 of length.
Load with 45 Acp dies, but the .45 Colt sizer is best for the sizing operation. See PET LOADS for data. Best results are with a hollowbase bullet. NEI makes one for the Webley, but my Lyman 450229 shoots amazing groups. I use 21 Grains of Goex FFF with the lyman bullet. Don't bother with Mil Surplus Mark II jacketted ammo, except the rarely seen Canadian issue. Accuracy is dismal, and the cases are berdan primed.