1. Redding 204 Ruger Type S FL Bushing die you will need to swap out the guts for for one from a 223 as the shaft for the expander ball is to small for the 6mm expander ball, order a tapered 6mm expander ball and order appropriate bushing. A call to Redding CS will help you with this swap. You might have to open the die slightly to clear the neck of the 6mm-204. I did this working from the top of the die with a tapered diamond grinding bit in my dremel, work from the top of the die so there is no chance of hitting the sizing part of the die. I did this and load 22-204, 6mm-204 and 25-204 with the same die by changing out bushings and expander balls.
2. Order a custom die from Hornady or C&H4D to your specs. I had Hornady make my first 25-204 FL die because Redding hadn't made the Type S die for the 204 yet.
PM if you want a couple of formed cases 6mm-204 cases to play with.
If you are thinking about this for a CZ527 you might not have enough mag length to load any long or tipped bullets.