If you haven't started yet - I have a mittful of Kennametal tool holders. They are around Rc50 hardness, and will eat up your HSS cutter. This job is an advanced job and needs a hefty mill to make a good finish. You have to use a carbide insert dovetail cutter after sawing out most of the metal using a bandsaw.
The expensive way is to ruin HSS tooling to do this. Even a standard carbide end mill will get dulled on this one job. If you have a 1" insertable mill cutter, and a insertable dovetail cutter you can get away with minimal tooling cost.
BTW I have a 1.5" Square Sandvik tool holder I intend to dovetail for my BXA, and I have postponed it for the moment. (I need better tooling before I begin)