68 383 with 750 carb. Runs rich

Backfires can damage a power valve and cause it to run rich. Double-check the float level and new needle & seats. The 72 primary jets are original. The secondary plate is equal to 75 or 76 jets. They make a secondary block that can replace the plate so you can easily change secondary jetting.

When going through a carb, I like to spray carb cleaner through every hole, especially the air bleeds (from the bowl side). Watch your eyes!

You might be able to get a little more idle airflow by opening the secondaries a tiny bit with a screw located underneath near the vacuum pod. This is like drilling holes in the primaries. But that's usually tried with bigger cams.