I have found that accuracy potential is
related to bore condition. It may not be the only story but it is major. I only have one of the old rifles with a sad bore. It is a Win 1885 in 32-40. I piddled around with it with cast and found that a fairly hard and heavier than normal cast bullet driven a little faster than normal let it shoot well. Cleaning (mining) lead after each range outing is still no fun.
I'm really just "shooting in the dark" as far as this design for the 405. I know a similar design in a couple of different weights has worked extremely well in my Sharps 45-70 and 45-110. My 405 shoots the 300 gr Hornady JFP really well and does well with a cast bullet I've gotten from Montana Bullet Works- probably similar to the one you're talking about.
I think you can still get yours to shoot well with cast. Just don't know about the tight spot. That may cause more problem than the general bore condition. I don't think there is a magic bullet design that would overcome some bore conditions??? Might just try something way different, within prudent pressure limitations and see what happens. Like different powder, bullet hardness, weight, bullet diameter and so on. Anyhow best of luck.