Looking for a little advice on the timing numbers I just pulled from my 73’ Duster (backfire issue)

Manufacturers set ign timing conservatively because they have to allow for the worst case scenario....which might be carrying a full load in summer heat up a long incline.
Hot rodders who baby their cars therefore have some latitude when setting timing.

The first line of the quote in post #9 is incorrect. Combustion is never complete at idle because of exh gas dilution ie, residual exh gas remaining in the chamber & contaminating the incoming fresh charge. Chrys DID use vac adv connected to manifold vacuum in the 70s to cool the engine at idle.