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i think the coil might be bad
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Coils either work or they don't for the most part. 20 Kv's is more than enough (spark can jump 1/2" from coil wire to ground).
LA engines are almost univerally 0.035" plug gap (use Champion copper plugs). Timing is generally 10 degrees BTDC (late '70's and '80's 318's are sometimes as high as 16). Be sure to check the condition of the cap and rotor. Also, '73 should be single pick-up coil; it could be failing, or improperly gapped from the reluctor (0.010"-0.012").
But make sure your problem is not mechanical. Broken valvesprings, burned exhaust valves, dead tappets or flat cam lobes will all cause a misfire. My '78 Apsen had a flat lobe on the #7 exhaust. Not a common problem for Mopars, but if there's lots of gumming on the tappets, they'll hang up and flatten cam lobes. Mechanical problems are easiest to find by checking compression. Confirm that the cylinder showing low PSI is the faulty hole by yanking plug wires one at a time and listening to see which one doesn't drop RPM or increase roughness.