I found this in a pamphlet among the papers of Max McClellan (the general's son) at Princeton University last week.
Some of his words in that address presage his son's refusal to engage his critics during and after the war:
If you have a low, crafty, and hostile competitor to deal with, let him alone and he will work out your reputaion for you. If you take the trouble to refute his calumnies too often, he will, in some measure, bring you down to his level in public estimation. ... A sensible author has somewhere observed that there are two ways of obtaining prosperity in the world - the first is by the praises of honest men, the other and surests is by the abuse of rogues.