I'd back off to 70grs of Triple Seven and see how she shot...I use 80grs of FFF Goex in my .54 and it's plenty for deer...Another advantage to real black powder is that it's ignition temp is lower (800 degrees F.) than the substitute powders...So it ignites easier and is less likely to burn through patches...

When I made my .54 flintlock back in the 80s I went with a Lancaster stock and a wide, flat butt plate...This coupled with the 38 inch barrel which is 1 1/6 at the breach and 15/16 at the muzzle make it a joy to shoot...

I started with 120 grs of FF and then went down to 100 and have settled on 80grs of FFF Goex in the past 10 years...Triple Seven is more powerful than real black powder so I believe you could go with a reduced charge...

Going to Goex might reduce recoil as well...

One way of looking at a round ball is that it's close to bore size...With a .50 inline and a .45 saboted bullet we are expecting the bullet to mushroom...That .530 ball mushrooms more easily because it's made of pure lead (not jacketed) and is also flatter at the forward section than a bullet...So even if it doesn't mushroom it is plenty big to be lethal...