At each box and some mid-point at couplings too. 400 feet between boxes with 3 inches per hundred, just at the boxes would not be enough. Anyway, you're gonna have to re-pull, and modify the whole run, why not start from scratch? As not only is the run on a chain link fence an eye sore, but if it doesn't have a ground wire in the run your fence is now the ground conductor, even then, still not hip. A closer building and underground sounds like a good perminant installation.
"Well - I oughta....." -Jackie Gleason