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

Mattax,
You can carry on until the end of time but what I said in post #20 is correct.
Combustion at idle is NEVER complete. This is due to contamination of residual exh gas in the com chamber; worse on low comp engines because the chamber is bigger & more exh gas left over. Anybody can prove the point by taking their stock Chrys engine & increasing the initial timing of 0 ~12*. Rpm increases. The rpm increases because with the lower timing, combustion was poor, but combustion is now improved [ but not perfect or complete ] with more timing & the engine makes more HP [ rpm went up ].

Chrys used MVA like other companies did in the 70s to reduce engine temps. You can blame emissions, hydrocarbons etc but the answer is far more simple. Same as the paragraph above: more timing [ added via MVA ] increased idle rpm, which increased water pump & fan speed & brought down engine temps.