will these fit on the back of a 69 or is there not enough backspace?

It will not fit properly. If it does fit at all your car may look like a ghetto cruiser (the wheel will protrude from the wheel well if it clears the lip) and the tire will rub the fender lip under heavy suspension load. I'd go with at least 5" backspace or up to as much as 5.5 if you have the clearance on the inner wheel well (measure it!).