Don't bother trying to set the XR400 up for mx... especially at your weight.
The frame flexes too much, the suspension is too soft, the steering angle is too steep, the wheelbase is short and the bike is heavy (top heavy to boot) so it will always wallow around and feel less stable on jumps and whoops etc than an mx bike. The faster you go the worse it gets. If you get the suspension set up stiff enough, the frame will start to bend particularly at the footpeg mounts. You will have a hard time getting the stock forks right for serious jumping. If you can get the handling somewhat setup for MX, you will promptly notice its lack of power and too-wide gearbox.
I went through this whole process with my first XR400 in 2001. After 3-4 months of expensive experimentation, I bought a used CR250. I'd say definitely buy a mx bike for mx, you will be glad you did.