Hi Phil, years ago I had a very nice rifle made exactly for what your asking. The offset in the stock was probably in the 6-8 inch range. It was an antique and I never shot it. It was very comfortable to hold. I don't know who you could get to make one. I would imagine the measurements would have to be pretty precise to make it work for you.
After developing macular degeneration in my right eye I started shooting left handed. I've been at it now for a couple of years. I went from being right eye dominant to left eye dominant.
Initially the transition feels awkward, but if you practice, you will get to the point where when your handling a rifle, you'll have a momentary lapse in memory and won't know what shoulder to shoot it off.
The most difficult part is developing the muscles. The mechanics of shouldering the rifle and sight picture come naturally after a short period of time.
It can be accomplished without much practice. You already have all the basics.
I'm at the point now where I have won a few local matches shooting left handed.
The expensive part is the re-tooling. I now have three left handed rifles in the arsenal.
George