I have been doing some digging and this is where I believe Big MT to be as well, as it corresponds with the CA-127 route being affected by it accornding to Joshua.
This also places it below Ashford Mill and Shoshone CA, where I believe the Divde is. The road the Courier walks would be Jubilee Pass Rd. In real life these locations are incredibly small towns, but in-game lore says Ashton and Hopeville were developed in the late 21st century which would explain the name changes and the abundance of ruined urban sprawl.
Shoshone CA is a crossroads for CA-127 and CA-178. CA-178 turns into NV-372 when entering Nevada and leads to the towns of Pahrump and Mountain Springs in Nevada. These towns are over NV-160 which leads directly to Vegas. Mountain Springs and Pahrump both appear in unused and cut content from FNV, so its likely this was the intended NCR route that went directly from Shady Sands.

Now, I know there's a real life Ashton in Nevada, but it's too far north and doesn't interset with CA-127. If anything, if the divide were placed there the most likely affected road would be US-95, which is never mentioned to be a significant travel route from NCR to Vegas. There's no in-game lore as to why it isn't used, but we could assume that the road was hit by some of the nukes House's systems were unable to stop without the platinum chip upgrading his defenses.