Sitting still the stock YZ spring is good with significant preload. I haven't tested it dynamically yet. Since then I picked up a YZ490 rolling chasis. That shock is the same, but the spring is nearly 1mm larger diameter. That should help quite a bit. Also I found progressive springs for the front that are the same outside diameter, about 12mm longer than the YZ. They have nea nearly twice the spring rate as the YZ.
I'll need to road test it before we determine springs, but there are options. I am trying to reduce the weight as much as possible at this stage.