I am in the process of just such a conversion. I have to say it was allot more work than I anticipated. I have done it several times before with no trouble and once even built a rifle that can be easily changed back and forth.
I wish you had a photo of the rifle with the barrel in it? One big problem can be that the drum is installed high on the barrel.
I am trying to attach a couple pics of the gun I am now doing. I didn't take notice when I took the jb, but you will see how things line up. The touch hole if I use a purchased liner will be at least 1/16" high.
That may present trouble, in fact the hole may be above the pan cover.
If he is just shooting in matches may not be a problem, but it will look crappy and I don't like folks thinking I am responsible for that.
I am going to try a regular vent and see how it works and looks. But may end up making a vent with the hole lower.
Allot more work.
Consider that, as well as the triggers, this gun needed work to make the triggers work.
Also they had beveled the lock bolster. I didn't do that I just glass bedded the lock level and tite on the barrel.
This is a target bench gun and there is nothing PC about it, I have no idea why they did that unless it was a used lock from a finer pc.
Just trying to help.
Have fun