So... I finally took the time to work on this. Last winter, my engine builder checked and replaced the diagonal pull rod. The one I had in the car was far too long (on the left in this picture, on the right is the new one):

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This made the pedal feel much better, so I drove the car like this during the summer, but I still didn't have full throttle at the engine. So last week, I followed Ed's advice to the letter. Started at the pedal, adjusted it to the stop, and worked my way to the back. Now I easily get to full throttle and all play has been removed from the linkage, it feels awesome. Thanks Ed!

The only problem that remains is that I hear a creaky sound from the tunnel when moving the pedal all the way down. And there's a lot of play in horizontal and vertical direction on the rod in the tunnel. I suspect that the white plastic guides are worn or missing. In order to get to these, I assume the handbrake assembly needs to be removed?