This is a perfect application for Buffalo Rifle (very close to AA5744). I use 25.5 grains under a 405 Hi-Tek coated cast bullet. Out of a Cavalry Carbine replica Pedersoli Trapdoor 45-70 I'm at 1200 fps. Your sharps has a lot of room to drop that because of the much longer barrel, maybe start at around 24.5 grains with a 405, I'd guess would get you near the same velocity on your Sharps. This is a < 18,000 psi load.
For CAS, it's my understanding that plated is not legal for most classes in CAS, but Hi-Tek coating is legal.
Unfortunately, I have never seen, or been able to find on Google, any Buffalo Rifle powder in Canada. And yes, I have confirmed that plated bullets are not allowed in CAS.