I realize that. The bore of the casting has to be a close fit with the liner or the liner is carrying all the load. By making the plug OD the same as the liner OD, the hole in the casting can be a single diameter instead of stepped. I would leave a short section for the weld at liner ID but make the remaining part of the plug that is out of the bore liner the same diameter as the OD of the liner.
I can add a drawing if there is interest.
If you're not a liberal at 20, you have no heart; if you're not a conservative at 40, you have no brain.--Winston Churchill