What is the roads closed got to do with it ? If you are the emergency services then you open them !
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?