Electronic devices may be the big pole in the tent in regard to this issue. When I was restoring my BOSS 429, I found that after 35 years most of the sheetmetal/mechanical parts were either readily available (for a price) or were being repoped. There are few electronic components in a BOSS 429, I can think of only two items - radio and rev limiter. There are hundreds of electronic components in a Ford GT. Electronic components have a very short commercial life before going obsolete - I believe these parts will be the most difficult to procure in future 10/20/30 years from now.