Slaves (of any race) were counted as 3/5 of a free preson, not 2/3, for the number of seats in the House of Representatives. The seats of the Electoral College are the total seats in Congress - that is, 2 (senators) + the number of representatives. So, yes, the 3/5 did effect electoral college representation, but not directly.
And of course it was the North that insisted that slaves were less than a person. The slaveholders would have been very happy to have them counted as full persons for purposes of representation, as long they remained property. It was the Northern states that insisted on reducing the South's number of Congressmen.