All the reports I have seen and the maps that WS'rs have made show his remains found in Harper Canyon, NORTH-EAST of Harper Flat, as if he was heading towards Highway 78. My guess is that it's about 2-2.5 miles from the presumed area of the truck, and it would be substantially down in elevation from the south end of Harper Flat, where the Fish Creek Canyon road becomes Hapapa Flat. There is a small trail to what is called Dave McCain Spring, which is a dry spring and actually has been turned into a "guzzler" to water Bighorn Sheep. I don't know if perhaps he was heading that way to find water, and found none.
Guzzlers are installed in wildlife areas to provide sources of water for wildlife, and are often present in BLM and remote areas of the west. In this area the are intended to provide water for Bighorn Sheep, but I have also seen them as resources for birds such as chukar.
I noticed it on the hiking trip report of the Harper Canyon trip:
If you look at the photo section, you can see two photos of this hiking group at the guzzler, photos of 11 and 12, taken in March 2018.
That hike circled around the guzzler and took photos of it (where it looks completely dry), although they didn't label it as Dave McCain Spring. I had been map searching for it much closer to the Harper Canyon wash and was surprised to see it was so far south. I'm just wondering if this is where they might have headed.