I have made hundreds of martini henry cases and snider cases from magtech cases and can tell you how to do it and where to get the tools etc. I do make them to sell but i am happy to share how its done with anyone who wants know. Sadly you do need an intermediate die and this is expencive. even if you buy a lee die from the states and bore it out.
Did you get round to necking down some brass and if so how did it go please?