Until it is running there is no way to make guesses. You could end up retarding it for no reason.

Ignition is an important area, and you really should seek out a capable tuner when it is running. You could try just retarding the dizzy by 10deg, but that is a lot, and I doubt you actually need to go that far.

There will probably still be mechanical advance weights inside the dizzy, and any competent tuner should be able to work with these to limit high rpm advance for an easy cost free solution ( other than labour )

Years ago, the common bolt on option was a double diaphragm advance unit. Vac advance as normal under vac, but it also offered a boost retard option. I'd say you'll struggle to find one of those these days though

But no matter what method you go for, it will need someone competent to check the engine is not detonating in any shape or form.