Metallurgist? I spent a few years in a research lab doing metal corrosion investigation, and while you're definitely right about the grain structure of annealed metals, I seriously doubt its a major factor in the production of the AL used in the chassis materials. To really screw things up you'd probably have to take it from just under the melting point and then quench it.
Chemical interactions and environmental factors are going to contribute to the aluminum deteriorating any faster than steel. If you want to last longer than just about anything, gold is the only way :lipsrsealed:... and that's not an option. If anything, you'll create a dendritic structure that is copper/zinc rich and/or it will corrode along the grain boundaries.
If you want it to last, park the vehicle in a temperature and humidity controlled garage, and never drive it in the rain.

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