I can literally watch other cars on the track gain distance as I change gear, and I've come to the conclusion that the major time loss is due to the very long clutch pedal travel. If I try to accelerate the movement, it only ends up with bad timing - causing wear on gearbox and loss of traction.

I've replicated the setup in this post:
http://www.teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1599

I haven't finalized adjusting the length of the rod, and I'd like to know if I'm within the ballpark; what final rod length did you end up with?

HI,
I did not know any Corvettes were in your country, plus those who would race them as they were designed for
Good for you !

I cannot recall the exact length since that was back in like 2006 when I made that clutch pedal stop for my 1999 Mallett 435 C5

I did use a bolt as shown below, it's length and then kept adjusting the length till it worked they way I wanted clutch release speed.
Looks like in the image I adjusted that bolt length down about 1/4 to 1/2 inch

You might also want to replace the clutch return spring due to it's age and also consider using one for a C6
Refer to :
Clutch Return Spring C5/C6

Also as i have done was to take a straight spring, mounted it to upper pedal arm and then to support under dash at a angle
That really helped at high RPM shifts where clutch pedal does not want to return when lifting off pedal

Sine I did that I have no problems doing 6,900 RPMs shift's like in 1st to 4th gear at drag races