So many times I found cars sitting behind houses and under trees with the same story, carb problems and no one could fix them. The follow up is that they were usually sitting on a slant and that would be the side that rusted out.MOST of the GT's I ended up with had 70K-80K miles on them.
The Solex carb theory is plausible. My first and second ones were a bear on certain brands of gas. The second one was the first time I did a Weber swap and it lived many more years until I crashed it.
all the wiring was eaten by mice, so i might go to the pertronix i had in the other 72An old school Allison electronics ignition, pretty cool. I saw them listed as Crane Cams/Allison electronic ignition on the old Opel GT Source catalog info, but never saw what it looked like. I assumed it was like the Crane XR-700, but your photo says otherwise.