I agree that the 65/66 radiator would not be a good option due to its smaller cooling capacity. In your case I would opt for a 69' radiator, which should be easy to adapt to your 67/68 radiator support opening.
One word of caution though about the 69/70 radiator. Make sure that the lower radiator hose is located on the same side as your water pump inlet. A 69' SB radiator would have the lower hose on the passenger's side, whereas a 70' would be on the driver's side.