It's ok, but 3/4-16 would be better in a 1 inch barrel.
Some barrel makers will tap a 15/16" barrel with 5/8-18 in .54 caliber.
You will only have a shoulder on the lands for the plug to engage. But as has been covered in some recent posts on barrel breeching, a perfect seal formed by the face of the plug against an internal shoulder, though preferred, may not always be necessary. Some feel the threads alone can form an effective gas seal.
