You should be okay with a 35 grain charge with that CVA. My kids and grandkids started out with an old .45 CVA Kentucky rifle that I shorten the barrel (from the muzzle) to 26 inches, and they've always shot 35 grains of 3fg from this particular rifle with never a misfire. I don't think I'd go much more below that charge of 35 grains since we're talking .45 -vs- .50 cal...
I'm not a fan of the bolster type chamber, but they are a solid piece of work after examining one that I took off another CVA .45 Rilfe when building my wife her little .45. I shortened this particular rifle barrel from the breech end / and drilled tapped for a breech plug and installed a side mount drum - than drilled and tapped for a nipple...
I was younger then, and these things came rather easy to me back then...
Here's a picture with a q-tip passing from the anti chamber to the Drum;
Just a side-note; I have that anti chamber in front of me now and the opening is a 3/16ths hole running 3/8ths deep - then a sharp turn of about 3/8" into the drum. You'd really have to tap the side of your barrel a 'lot and with fine powder to get powder into the (less then 1/8" nipple cap channel to the Anti Chamber). I don't like the setup, but it is a solid one built to take pressure for sure.