Updated 6/14/2019:
Finally got off the dime. After an interminable number of measurements and re-measurements, I finally traced out the sub-roadbed piece connecting the eastern side of the Stone Arch with the rest of the layout (WV side). I haven't made the last cut, but here's a test fitting.
Construction of the sub-roadbed support structure is next, but this is a start.
More when I know it.
George