You are looking for parts that are not used on many Mustangs. The "big block" 429 was last used in 1971, and big for 72 and 73 was a 351C engine. I think you are looking for 460CID parts and there is the problem. That engine was a truck engine, but used on 72-74 T-birds, 73-74 Fords and 74 Torinos (and probably a few years after that). It is a derivative from a 429 so that's your base. Around that time, "big blocks" were out as fuel economy became the focus of car manufacturers and government. You want parts, there's your range: 1971 Mustang 429; 72-74 Ford 429 and 460. Performance parts for your car are best obtained from Summit Racing or Jegs. There's Craig's list and e-bay too.