I guess it depends on your definition of "custom." If you mean a builder who will start from scratch and build something entirely from your design, then I don't think GXV qualifies. They have several basic designs that can be modified somewhat to meet your preferences.
GXV works with a number of truck chassis, including Ford, Fuso, International and several types of Mercedes. GXV gives you more choice in chassis than other USA manufacturers I'm familiar with.