I've been collecting the stock parts to convert mine to the 68 Throttle Cable style linkage. Or you could always go with a Lokar or aftermarket cable set up if you didn't mind going that far from stock...

I've had the same issues and have also had the linkage limited in travel by hitting the back of the head... hence getting ready to change it out the next time I pull the motor for detailing... which may be soon.

Brett - 67 Drop Top 455!