ND 0.3 is only going to cut down 1 stop, probably not enough to stop the ugly over exposure outside. You only have 4-5 stops of latitude to play with on the 5d before you start losing detail rapidly. LED's are soft lights anyway, if I felt like it needed to be softer I would only put 1 layer of hollywood frost for more diffusion, anything more than that and I would be cutting down light unnecessarily. Remember the suns going to be lower in the morning! Hope you have a great shoot, post the results!!!
P.S Litepanel lights come with hard gels, consider putting some minus green in (if you decide to go LED).