It appears that transverse loads are all concentrated to the point in the center of the rear of the modified brace under the exhaust. Any excessive lateral compression flex of the top X brace will transfer the moment loading to the lower ends of the vertical solid aluminum chassis members on each side. If allowed to continue this will more than likely result in weld failures at the point where they are joined to the lower horizontal fore aft chassis tubing. This is the purpose of the relative square and cross-braced design of the OEM brace. The transverse front section on the original design is nearly removed and the new design looks to focus load paths nearly directly to the 2 rear top corners of the total rear chassis box instead of all four. You apparently added the small curved tubing section under the collector to try and mitigate this but the resulting X brace shape is now more of a "U' instead of an "X".
The four top ends of the vertical chassis solid aluminum elements cannot be allowed to move as a result of loading. If they do the welds where they connect to the horizontal chassis elements at the bottom will be forced to absorb the energy instead of the entire structure, resulting in weld failures at those points. Adding a horizontal cross element between the front two vertical members will correct this issue somewhat and it appears you still have room for one.
Additionally, it appears that the rear triangularization links between the middle of the top rear horizontal X brace tube and the lower corners of the chassis have been omitted. IMHO you should at least add them. Without them then the whole rear chassis box has no real triangularization and will result in weld failures at the same points as discussed above. I would consider this imperative.
I hope that this is received as it was intended and not as a criticism. I kinda held off with this because I don't want to offend you, I really like the work you have done so far and I can't wait to hear it run.
My best, Howard.