The Mad Hat Man said:

Audi ASF
Audi A8 is the first mass production car featuring Aluminium Space Frame chassis. Developed in conjunction with US aluminium maker Alcoa, ASF is intended to replace conventional steel monocoque mainly for the benefit of lightness. Audi claimed A8's ASF is 40% lighter yet 40% stiffer than contemporary steel monocoque. This enable the 4WD-equipped A8 to be lighter than BMW 740i.
ASF consists of extruded aluminum sections, vacuum die cast components and aluminum sheets of different thicknesses. They all are made of high-strength aluminium alloy. At the highly stressed corners and joints, extruded sections are connected by complex aluminum die casting (nodes). Besides, new fastening methods were developed to join the body parts together. It's quite complex and production cost is far higher than steel monocoque.

The Audi A2 employed the second generation of ASF technology, which involves larger but fewer frames, hence fewer nodes and requires fewer welding. Laser welding is also extensively used in the bonding. All these helped reducing the production cost to the extent that the cheap A2 can afford it.
Advantage: Lighter than steel monocoque. As space efficient as it.
Disadvantage: Still expensive for mass production

The main problem with Aluminium is that it has relatively low fatigue strength and unlike most ferritic steels it does not have an "endurance limit". The endurance limit is the stress at which fatigue failure will never occur. Aluminium will always fatigue even if stresses are very low.

Unless the whole concept is studied and analysed in detail I feel that an aluminium copy of a chassis could be bloody dangerous. The impact loading on pick ups would need to be modelled and design changes made. Some maths should be used instead of "it looks OK". I think this would be an interesting but time consuming process material data and mathematical models required. The welding control that would be required is very advanced and quite costly and I would advise a manual TIG system with an HF supply and a "crater elimination" unit. The correct shielding gas would depend on final alloy selection.

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