Almost Eureca, almost Victory
With the old ignition system, the engine runs well on all 8 cylinders! There are still some backfires at startup and if I want to reduce the idle too much, below 1100 rpm, it messes up, then a big backfire and it stalls but with a cam that has 60° crossover and vacuum advance which no longer works (the old membrane of the old igniter is pierced), this is perhaps normal. The spark plugs, even the new ones, worked with poor ignition and some may already be more or less out of service, I will order a new set.
So either the Pertronix box is malfunctioning, or the spark plug wires are not suitable for too powerful ignition and create crossfires. I'm going to start with a new spark plug wire harness, I'm thinking of buying these:
https://www.summitracing.com/parts/pnx-808280
because I can buy them from a seller in France.
And if it still malfunctions I will just have to change the box wich is brand new but as I bought it almost 5 years ago, there is no point in hoping to have it changed under warranty.