Sounds like your muffler shop goofed. Take it back and have them fix it. I had duals put on my '83 Merc Capri. The muffler shop stole my California Turbos, broke a break line, and burned a hole in my fuel return line. Idiots!
The "heat flashing" is the metalized shielding to protect the gas tank from direct radiated heat from the exhaust. Remove it and you could go boom one day.
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