I an contemplating modifying my NN3 to accommodate a D300. With my Kirk QR plate and clamp on the upper arm, the lower rail setting is 7.5. I need around 5 mm more, which means either grinding away 2-3 mm of the outer edge of the bottom of the vertical arm, or grinding away the flange at the end of the lower arm. I think the first option is better because grinding away the flange might compromise some strength. I think the NN3 should be able to handle the D300 weight wise and would prefer to keep it rather than spring for a NN5.