Tubular powder will burn faster because it has a bigger surface area than a flat disc. It does not matter which powder you choose. The loading data will give you a start load and maximum load, which must not be exceeded.
You might have to try several different loads before you find one that suits your gun. When it is a good load you should be able to shoot clover leaf groups at 100 yards.
The bullet has to match the rifling twist. The faster the twist the longer the bullet has to be. I can't remember off hand what bullet is for the 1:8 twist. I'm sure someone will tell you if the 55gr is not right.