Two things that come to mind:
1) Just by the nature of the material, regardless if it's 6061 or 7075, the two most common/available types, aluminum WILL fail. No matter how much or how little the loading is, it will eventually fail in fatigue at some point in its life if cyclical loading is applied to it, such as road vibrations and hard corning.
2) With that much fabrication and welding, heat treating the frame will be a must to regain uniform strength in the heat affected areas of the aluminum, or else the parts of the frame not affected by weld heat zone will be stronger than the "soft" welded areas. The frame can be allowed to sit without being loaded for a certain period of time to let it age harden to regain more uniform strength, but for a high-load application like a pro-touring car, and especially with the NVH from bumps etc, the aluminum would be better off heat treated. Maybe you could pay a local powder coating place to use their oven, but might be expensive cuz heat treat can last up to 10-12 hours depending on the alloy. I believe 6061 needs much less heat treat, but I'm speaking from experience with 7005 which is a pretty rare alloy.
If neither of these scare you off.. go for it and good luck dude! But yes, definitely speak to a metallurgy/structural engineer to find out the more specific details.
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