You can safely plant out gallon sized plants now. I might wait and pot up the quarts for a season before planting them out in late summer/early fall, but it would depend on the specific vine and how well-developed the root system looked......personally, I feel larger vines with well-developed root systems just have a better ability to establish quickly, but it's your choice.

Read up on the info on preparing a proper planting hole for the vines, the need for planting deeply and suggestions for first season pruning. This advice is all intended to encourage rapid establishment, the development of a healthy root system and reduce the incidence of wilt issues. Attention to watering will be critical their first season in the ground. And I would NOT recommend dividing. While some have had success with this method on a few limited types, it is not a typical practice for clematis and you do risk injuring or destroying both the division and the mother plant. If you wish to propagate, layering is a much more successful and risk-free method.