Disc options for 73-76 drum spindle???

Hmmm, interesting kit, but I wonder why they use truck discs and a spindle/bearing adaptor? Aren't the later B body rotors a straight fit on to these spindles? The AR kit looked better, but Viper calipers aren't cheap either, any other ideas?

Truck rotors are 1.25" thick instead of 1". That's also the same thickness as viper rotors so the pads are proper without spacers.