I certainly am no expert, but agree an outside thread is preferable. There are specialty gun smiths that offer the solution to your problem, but I personally cannot recommend any as I have no personal experience.
While the experience on this board is amazing and I am constantly reading on this board and learning, this may be a case where a specialty board can offer more. Take a look at Silencer Talk.
I was recently looking to thread a 10/22 and preserve the front site. I found the smith I needed on Silencer talk. Although, the Firing Line never ceases to amaze me with the depth of knowledge and experience. I will however also warn you that many choose to only interject their (hopefully) experience and will never directly answer your question. As I just did in the above post