65 - 68F is probably OK for carbonation, but it will take a bit longer. However, I question your assumption that your basement really is in that temperature range. I can't imagine that your basement temperature would be very different than mine, but last time I measured it (about 1 minute ago) it was 57F. I put the thermometer in a case of beer that was resting on the floor. 57F is probably too cold for carbonation.