Levers are great. Why do you want/need to co-witness? I know it is all the rage in tactical circles these days, but since you are looking at a lever you are not really swayed by the prevailing AR wisdom.
I would look at a Burris Fastfire. There is a pic of it mounted on a levergun here. In the mounting instructions, they specifically mention that it does co-witness with the irons, as that is how you do a rough sight in with it.
http://www.burrisoptics.com/fastfire.html