63/66 Valiants

I would say the 63 firewall (without the distributor kick out) is the most obvious change in that span (uni)body wise, but I wouldnt even say 66 was in the same catagory front clip exterior wise.. Different bumper, fenders, grill, headlights, taillight shape...
Computer terms:
63 was version 0.9 slant only
64-65 was 1.0 V8 ready
66 was 1.5. front clip change
67 was a 2.0 entire revamp with wider frame rails.