Nice write up on the carburetor work you've completed.. I'd not seen that blog.

That said... to quote you:

"Lesson learned - it's impossible to tune SU's without having the rest of the motor in good order."

So, that's a good place to start, and that PVC thing sounds amiss.. I would pull the line at the manifold and plug the hole in the manifold.

Then, Get it running as good as possible, and go through ignition again. Mainly, check points and timing again.. another quote:

"6. timing - OK, I fibbed. I still need to get my hands on a timing light."

Did you get your hands on a timing light?

After knowing that all issues beyond the carbs are good, I would grab your Haynes manual again, find all pages that relate to carburetor tuning, tear those pages out and throw them away.

I'd say considering all that you've done, you know your carbs better than the book... Fine tuning is almost always done 'by ear', 'by butt', and by fuel gauge. (that goes for timing as well, but it's good to have a light to get it in the ball park)

I don't like the '21-25 flats' to get to that .06" baseline figure.. I'd be suspect of your measuring technique.. perhaps you aren't getting the back of those calipers quite flat... If you are dropping them down 2x the 'normal' baseline, that'd cause your problems for sure.

Also, lifting that piston by only 1/32"... I'd think that's rather small. Heck, the added air allowed by that much is offset by the width of the screwdriver you used to do the lifting..

Look for fuel leaks... even if running horribly rich, your car shouldn't smell like fuel. If it smells like unburned hydrocarbons from the tailpipe.. that's more likely running rich, but that's not quite what you said...

You'll get it, and you know that.. Best of luck!

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-Matt I ♥ my ♂