Typically if someone is custom-building a suspension for their truck, I normally still suggest they buy a set of good sturdy axle brackets from a reputable manufacturer (such as Skyjacker) as it seems this is the easier way to do it due to their asymmetric nature and that of the axle mounting arrangement. The entire rest of the suspension (radius arms, coils/coil mounts/shock mounts/coilovers, steering, etc.) can then be built around those brackets with relative ease.
To answer your Q about bracket material thickness, typically 3/16" CR plate is used in the aftermarket, and occasionally even 1/4". Some companies cast their bracket for the passengerside beam (much like how the factory piece is, just longer & thicker).