Pull the FW tires up onto lumber until it is level. Then measure how much rail clearance it has. I think most people want 6 in or more. Just say it was at 6 in then measure the amount of wood under the fw tires and that's the amount of lift you need to come up with to level the fw.
If the rail clearance is not enough then you need to adjust the pin box and add even more lumber under the fw tires. It's not unusual to need an axle flip. If that is already done then you may need a sub frame. You need to come up with the numbers and then look at your options. Ideally you would like to be near your final weight when doing this.
Larger tires, correct track, minor shackle lengthen, axle flip, sub frame and some will even suggest a spacer between the spring pack and axle when they have already been flipped. I wouldn't use those but some do.