I want to drill out and tap a couple of Ashley/XS ghost ring rear sights to take standard Lyman/Redfield sight discs. What is the correct tap drill and tap size?

Why not just buy the threaded post they sell. Less than $10 from Brownells. The thread size you are looking for is 7/32" x 40 tpi. If you D&T one of the smaller aperatures my impression is that you will end up with very thin walls.

XS/Ashley does not offer threaded posted in all heights. I bought one and it was too short. I had to send it back. I have two rifles that require posts taller posts. Been there done that! I have done some mesuring, and there is plenty of stock to drill out the hole and thread it.