Slant6 specs questions

Been spending time going through the car, 73 plymouth Duster, cleaning it all up and getting what I need to get it started for the first time in decades (taking time to try to do it all right) and have noticed a few things. It's all original numbers matching 225 slant 6 car, It was built in September of 72 as a 73 model year and my understanding is that the 225 car was a 4 wheel drum brake car but it has disc front breaks. Would that mean that it has a beefier k-frame or other equipment not standard for a 225 car to accommodate the disc brakes? Also to my knowledge standard for that year slant6 car were 5 on 4 wheels but this car has 5 on 4.5 wheels. Were these paid for up grades or standard on some cars? Still trying to figure out what I have sitting in the back yard. Eventually years from now I want to make it a v8 car.
Thank you!