I don't completely understand the total problem but have a few suggestions.
1. You need to have the structure in equilibrium. Grounding the square tube and applying the boiler load and a > boiler load to the fork tube will cause extra bending in the square tube.
I would include a model of the forks - use contact between the forks and your structure. Use a displacement boundary condition at the root of the fork to raise/tilt the structure like the real lift.
2. I suspect the physics are only 1 plane of symmetry and not 2.
a. Common for forks to not extend through the structure being lifted. Also forks are tapered.
b. Fork lifts almost never raise straight up (they tilt to get the load closer to the lift). this angle will change the center of pressure and put more load on the square tube near the forklift.
I suggest using at most 1 plane of symmetry (use a split line between the forks). For this problem it may be best to just run the full model.
3. The boiler load is not likely to be uniform on the upper rail. It will mostly likely concentrate on the vertical members. This will reduce the bending in the verticals and the gusset loads on the square tube. You may need to add some more of the boiler model to get the right distribution.
can you draw up a diagram of the forces and how things are restrained in "real life"
it would also be helpful to note where you are expecting the stresses to be and why you think they will be there.