why cant we use B or E front disc?

Thanks for all of the info everybody! My brother and I are putting together a 1971 dodge dart street/strip project and I've been wondering about the front disc brake conversion. We need to stick with manual brakes because we're planning on running the hooker super competition fender well headers and they will not work with a brake booster. Thanks again for the great information! I hope that everyone has a great day!