No need for a filler with the following loads with RL#7 which by the way happend to be one of my favorite powders for that cartridge.
These loads are Trapdoor level at a max of 16,000 CUP
Min 35.0 gr approx 1300fps Max 37.0 gr approx 1450 fps
These loads are 1886 Win and Marlin1895 level and should be no problem for your rifle max load generates 26,000 CUP
Min load 41.0 gr for Approx. 1600 fps
Max load 46.0 gr " " 1800 fps
BTW If you have or can get powders like 2400, H-322, N-133 or 2015BR they are excellent alternatives to RL-7
For real low velocity and recoil SR 4759 is hard to beat
I have loaded as much as 50 gr RL-7 to push a 350 gr Hdy and 400 gr Speer bullet from a Handi and found recoil "memorable"
and decided that there was nothing where I lived on four legs that I needed to confront with that load!
BTW The Speer Manual shows hotter loads for their 400 gr FN with RL-7 starting with 41.0 gr to a max of 45.0 gr for Trapdoor level and 49.0 gr for Lever Actions. Their top bolt action load is 52.0 gr