I know the FB hangs low on my trailer build,are you able to make a carriage under your FB instead of welding to the sides? More labor intensive I know but could give you the support your looking for.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford
If you're going to weld nice and strong on the CC side of the FB onto that cross member the way it goes to the FB shouldn't really even do anything so it would look like it was but probably not really ever have a load weld away I say.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
I agree with RC, but if you're nervous about it you could always weld a thicker piece of steel to the outside of the FB going from front to back & weld the supports to that to spread the load.