Since you're going to be drilling and tapping for the liner, you can choose your thread style. If you're not going to make your own liner, I recommend you consider the RMC replacement liners that are installed with an allen wrench. They have four or five thread styles avaliable. Check them out at:

http://rmcsports.com/catalog.htm (click on "blackpowder shooting accessories, and scroll down to touch hole liners)