John,
Your geometry was relativly easy to sort out. The two links are just about of equal length, r3 and r6 at 87.47 and 87.54mm.
The central shaft starts at +34 degrees (from vertical, +ve being clock-wise, viewed from the back of the engine) and runs through to -34 degrees
The webers rotate through 72 degrees starting at 36 degrees (standard weber 72 degree full travel).
The maximum difference of the two weber arms is 0.0188 degrees.
So you can use the original weber arms, and because your central shaft arm length is nearly the same as the webers there is very little error.
Remember that this is NOT allowing for any tweeks of the weber arms for tuning and idle speed, but with your set-up the weber starting angles are not to critical.
If you do stick with the original weber arms watch out for where you put the throttle cable, originals had a canted arm to give space to feed the throttle cable in.
Interesting to note that your lateral weber spacing is different to mine.
Hope this helps.
Andy