Welcome to Websleuths!
I don't remember the exact mileage but the phone call was made in Arizona north of Kingman (according to cell tower hit). My best guess is somewhere around the Dolan Springs/ Pierce Ferry turn off.
If I find the testimony I'll post it here.
It was discussed that the gun may have been thrown over the dam. I thought of the Tillman bridge also, but was informed it wasn't built in 2008, which I had not thought of.
HTH
You may be quite correct re the timing of the PT Bridge. Nonetheless, she did have ample time to throw the weapons off the dam.