Various resons why Ford didn't install the lower cross brace on the 63-65 Falcons:
Ford U.S. was still learning about unibody construction. There only experience came from the British Fords which had only had unibodies for about 8 years.
The cost. Think perhaps $5 per car times tyhe number of V-8 Falcons built in North America.
The thought that no one in North America would drive a Falcon "hard". The U.S. had mostly paved roads, Canada and Australia/New Zealand didn't. Aussie and NZ Falcons had cross members.
No one gave thought to the fact people would still be driving unibody Falcons 40 + years later! Even the today's well-preserved Falc's aren't as strong as they were when new.
But Ford has learned a lesson, notice the upper and lower cross members used in the new, hot Mustangs? Their effort is worth duplicating on old Falcons. | 
