well i think that, although tactically the TL's would be different. The overall picture would be the same. Richmond would fall, and Lee would be hard-pressed for a victory, or else the Eastern front would literally collapse on itself. Lee cannot keep on the defensive. McClellan would just do as Grant did in 1864-5, which is keep out-gunning Lee. Id see the war in the East just give out by late 1863, with McClellan and Sherman possibly meeting in Georgia. From then on the CSA has no industrial base or manpower to stand on. After the war, instead of Grant getting the gold star next to his name it would be McClellan. Also, Reconstruction would probably be a lot less painful, maybe even letting some states start gradually emancipating slaves. Depending on if they surrendered the easy way or hard way of course.