.54 34 5/8 1-65 twist That would make it a roundball barrel.
Rifling may be too deep to properly seal a conical,that rate ot twist is slow to stableize a long conical
May work with a shorter conical around 300 gr may have to experement with & without over powder wads.
With a shorter conical may shoot good , bad or ugly or just maybe realy good.
with a short conical you would still need a heavier load , the faster the bullet is going the faster it will spin, the spin would be needed for stability.
I understand that you are looking for someone who has tried it , so far nothing.
Now that the rate of twist has been established someone who has tried something on a like or near like barrel may have some report to how it worked instead of just theory.
Your english is no problem,sometimes you need a little more detail, untill I saw Pedersoli I had no idea just what you had, one word can make a huge diference.
Add to your information location Croatia, it will appear on all of your post.