That's pretty big for rabbit, unless you head shoot you are going to likely wreck a good bit of meat. Most primitive shooters I know who hunt for rabbit use something smaller.
But, it will work, as far a load would go as low as you can accurately head shoot the gun at hunting distances, hard to know what that is in your area. I would start at a max of 50 grains and see what that does.
I good luck and have fun.