I have to agree with Jerry and a couple other guys. You don't even need a gusset. 2 x 4 x 3/16 wall and assuming 16 feet long and 2 pieces , you are into about 200 pounds of steel just for the main rails!! Now you should be able to make a sound weld on 3/16 tube any day of the week with either Mig or Tig. You are not going to break it with 500 hp in any kind of chassis. Don't forget you will have some kind of integrated roll bar/cage that will add emensely to the chassis rigidity, not to mention a pile of weight unless you use moly tubing.

I'm reassembling my hotrod in a racecar shop where they are building a FC with moly tubing according to SFI specs. It won't weigh much over 150 pounds complete and it be fully capable of handling 2500 hp every week end. The mate made over 100 runs last year and not a single broken weld pluss the car is 10+ old and the new car is better built than that one.

My street rod has 2x3x1/4 wall tube chassis and 450 hp at the rear wheels. I put over 1000 hard miles on it last year and nothing broke or bent.

By the way nice Cad models. SolidWorks?? If so why not run Cosmos on it just for fun. I have it but it is tricky to set up the restraints and there are too many unknowns for me to do it.