I have a Mark VI. I have altered .45 Colt cases to .455 by shortening with a small tubing cutter and trimming with a LEE hand trimmer for the.45 ACP. The rims have to be thinned to .035 with a file (the hard way!) or get someone to lathe turn them (the easy way!) This approximates the Mark I case at .89". .45 Cowboy Special cases with the rim thinned might be a better way to go.
HORNADY has made .455 Mark II cases, and you might ask around. Mark I are the longer ones (.87") and Mark II the shorter (.82"). Fiocchi makes loaded ammo. Military ammo is a waste of time! POOR accuracy and Berdan primed!
I have a heavy Keith type RCBS mould, about 270 gr. in W-W metal, which is close to the factory bullet, but I have used the 201 KT with some success. The best was the Lyman 450229 155 gr hollow-base. For a copy of the original heavy hollow base bullet look at NEI.
Cartridges of the World, 3d Edition, shows the following in the Mark II case;
250 gr bullet 454424 over 3.5 to 4.0 gr BULLSEYE (vel. 710 to 735 fps)
260 gr bullet over 5.0 to 6.0 gr. UNIQUE (vel. 610 to 710 fps)
Rattlesnake Jack has a lot of old Imperial stuff and probably uses it a lot. I got my old Wobbly talking, and then moved on to my competition guns.
As my revolver shot high, as they generally do, I like a lighter bullet over about 3.8 gr BULLSEYE in my .45ACP length cases. For Blackpowder try about 21 gr. of FFFg