Updated 6/16/2019:
So, the test fittings continue. First, I used a grinding wheel (with my wife's help) to make the western edges of the sub-roadbed align with the end of the Stone Arch. By propping up the sub-roadbed, I was able to conduct a test fit.
After that, I marked the spots on the existing sub-roadbed (already attached to the benchwork) where this piece will mate up.
Cuts will be made that allow the new piece to fit into the existing benchwork like a piece of a jigsaw puzzle. At least, that's the plan. 
Once that's been done, I will screw down the new piece and focus on creating a support structure underneath it. And there's the matter of mating the support structure to the east end of the Stone Arch. That must be anchored in a way similar to the Steubenville bridge abutment.
More when I know it. 
George
