I second this, grind off a bit of the head so it clears, and hopefully it makes it out through that notch

Make the tensioner the last pulley you put the belt onto, no ridge(s) to battle with. Also possible you have the wrong belt (even though you state you have an 875 even with the old was an 870). If it just won’t go with the tensioner fully slacked then it’s routed incorrectly or the wrong belt