The Modern Trail:
These days the trail has been upgraded so that it is now (occasional landslide aside) paved all the way from HCMC to Hanoi.
It follows what is now Route 14 in southern Vietnam and in the north it branches - the west route and the east route, Route 15.
I followed the more remote west route, except for a section around Danang and Hue where I left the HCMT all together and dropped down to the coast so I could drive on the scenic Hai Van Pass.
Here is the route I took: