That's a standard HEI distributor. Timing rarely causes backfire. The most common causes is crossfire between wires, or LEAN carburetion. If one whole bank is dead, that indicates carburetion. One main/idle jet is clogged.
Timing should be 30-34 degrees, all in by 3000 rpms. This is usually 8 degrees initial, depending on the distributor. Counterclockwise advances the timing.
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