with our MTS +3 kit, you can use the stock length shocks. Only problem is, they are going to be very soft. Most likely, you will have to get stiffer springs and stiffer valving to make them work properly. I know there are companies out there, than can change the shocks you have to make them work. just no sure exactly who can. You are probably going to run into this same issue with any kit that works with stock length shocks. The extended a-arms put a lot more leverage on the shocks, so it makes them feel a lot softer. I don't think anyone makes a +6 kit that uses stock length shocks.
thanks,
Greg