What #90 GTSC mostly sounds right. Minor point of clarification, the top longeron will be in tension (Not shear) and so would the point where it meets the main box. Having the top longeron be all one piece would be nice, but I think properly made welds with reinforcing plates should be at least as strong as the base metal.
The bottom longeron will be in compression which means you need to support the point where it meets with the main box. A diagonal brace from the floor of the main box to the point where the bottom longeron meets the box should do nicely. That would carry forward with a diagonal from the bottom longeron where it meets the main box to the top front corner of the entire structure. That would put both of those diagonal members in compression as #90GTSC suggests.
#90GTSC did get me thinking though. You should not plan to carry a lot of weight up there. I am less concerned about the structure, static or dynamic, than I am with overloading your front axel. You could compensate slightly by putting heavy stuff behind your rear axel, but the lever arm to the rear axel is long so the benefit not so great.
Good luck.
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