One way to get started is get a 6mm Hornady Universal Neck sizing die and then a Vickerman 6mm Seating die.  You probably need to size the 30/40 brass down to 7mm first.  I had a custom bushing sizing die built for my 7mm Krag Improved and use a Vickerman Die for seating.  One thing to be careful of if you find a die set is that they are for your length of 6mm Krag as there were at least two lengths of wildcats.  Tom