OP, before you load up 300 rounds in the FTX brass, make sure they'll feed through the rifle at that length. Some lever guns can be fussy with shorter ammo. Owners of .44 and .357 magnum lever rifles have sometimes complained that their guns work fine with magnum rounds, but choke on ,44 and .38 Specials. Good luck.
The .444 Marlin (or maybe a Winchester) rifle is on my wish list. I've managed to acquire a set of dies and a little brass, still looking for the right rifle.