I would suggest checking on his rank at various time periods during the war, and alter your impression so that the rank matches the time period of the event. There is someone around our area that does a Sherman impression, but he wears a rank that Sherman did not have until after the war ( and there are many of us that have noticed that notice that, and admittedly it lessens our respect for what he does). I think that when doing impressions of high profile officers, matching the rank to the time period is often overlooked but should be a part of the impression, since in most cases they progressed in rank during the war.