I was not convinced that machining the pedal box casting to fit the master cylinder, and needing a custom over centre spring was the best solution so I thought of an alternative.
I simply rotated the entire master cylinder assembly out of the pedal box casting so it would sit on top, and re-shaped the clutch pedal design to fit.
Inspiration is mainly from Audi A3 / Golf clutch pedal assembly, I bought one of these for £20 off ebay, and it came with a over-centre spring too.
In up position the clevis meets the pedal at 85 degrees, this seems to be how the stock LHD clutch pedal is set up. Master cylinder moves through 30mm, and is 75mm from clutch pedal mounting centre, so I made the over-centre spring arm 75mm too. This is how the A3 pedal was set up too, master cylinder mounting point was 58mm from pedal pivot point, and that was more or less the lenght of the lever arm going to the over-centre spring.
New list of requirements: custom clutch pedal (with polybush bushing), custom aluminium mounting bracket for master cylinder and over-centre spring (welded or bolted to pedal box), move brake pedal spring mounting to right side of well, custom mirror image brake pedal spring, ebay M8 clevis.
A few potential issues:
1) over centre spring is just underneath the master cylinder, may need to machine the pedal box casting a little so it can move freely.
2) over centre spring is 30mm wide, it will fit between the brake booster piston boot but with only a couple of mm clearance.
3) the re-designed clutch pedal reaches up 11mm above the height of the sensor bracket. I have had a look in the car without removing the trim and it seems like there is enough space up there, but not much.
Here are some pictures of a wooden test pedal, A3 over-centre spring and S8 stock clutch master cylinder.
