Just purchase a 69 Camaro with a 355 and a slightly hopped up cam. Car started backfiring very bad. Had the timing adjusted and it was still backfiring but only on the passengers side. Replace passenger side plugs and problem went away for a while. Would going to a HEI system provide a hotter plug to help this or what is the problem.
Check the Cap and Rotor to see how it looks. Clean the tabs inside the cap if it has deposits. Also check the spark plug wires to see if they are any good. Sometimes they get burned or something and they don't function properly. I would also check the compression on the engine to see if they are all equel or if there are some cylinders worn more that others.