Yes perhaps low flying fighter-dive bombers but bombers flying at 6000+ meters won't get reached by "aa-machineguns".
True, but smaller caliber AA cannons would have done the trick. Not to mention that by the time of the Great War, the Fallout world should have had AA missiles, perhaps even AA lasers.
And the higher the plane flies the slower it will apper to be moving making tracking easier
True, sort of. And a bigger shell would give a bigger shrapnel spread area, making it more likely to hit the target.
But consider the reloading time for that monster. A single plane rarely gets sent to attack a strategic target; the reason low- to mid-caliber autocannons are used is because of their ability to reload and retarget themselves quickly enough to take on several targets in succession. That monster cannon at Hoover Dam must have each shell weight 5-7 tons at the very least; there's no conceivable way to quickly reload something like that.