On here it is not usual to make one of Brads designs in aluminium .
The main reason sited is that the act of welding changes the structure of the aluminium at the weld site and to avoid these areas failing the whole frame needs skilled heating and cooling to achieve maximum strength.
If you are after the perceived weight saving , it tends to be reduced in actual life as due to aluminium's lack of fatigue resistance it tends to be of a heavier section than the equivalent steel
However we are open to anyone coming along and proving us wrong , however we expect to see photographic proof
IMO, you can build an aluminum frame bike without heat-treating using 7000/7075 alloys but the material here in the USA will cost 6-8 times what steel will cost. Using .049 wall round 4130 steel will reduce weight and if engineered correctly will be light but then you are still dealing with higher cost material and welding. If weight is really important then go carbon fiber but be ready to really open your wallet. Building a bike from the AZ plans will produce a long lasting, proven ride.