Comparing the pictures of the accelerator pedal rod of yours with mine, I believe a lot of your issue is the rod not aligning with the cable. By the time the slack gets taken up with that clip starting almost perpendicular with the cable, you have pushed a lot of pedal with little cable movement.

Also, there were two different throttle brackets in '68 depending on the intake manifold part number.
The 9790140 intake uses 9792242 bracket while the 9794234 intake uses an F4 bracket.