Way too many variables to call a winner here. Depends what cams, depends what headers, depends if the timing curve on both cars are set up correctly and the driver is a huge deal in these matchups.
I would bet a no option 440 automatic 67 GTX weighs within 100lbs of a small block 4 speed 71 Cuda but that could depend on options and how much bondo is in each car (seriously).
If both cars were stock I would bet on the Cuda if I was driving it, the 71 340's were the best year and the 67 GTX was hampered by a too small carb, a 71 340 Cuda 4 speed with 3.55's could touch a 14.0 if tuned and driven correctly, a lot of stock 67 GTX's were at 15.0

Sheldon