Because if you have evacuated an area because of the risk to life from the dam bursting, you would look pretty silly sending a load of civilian lorries inside that area if the dam did give way - it's just a sensible precaution you are happy to ignore to try and support your point about using a 350.
As Chinook 240 has pointed out - will you manage 24-hour ops in poor weather?
The map in post #23 shows the pick up point is a couple of miles outside the evacuated area, and at a higher elevation. The photos I added show a road across open moorland, no trees, no street lights, no utility poles - looks like as good a place as any.