back firing trouble

OK, so about 10 years ago my brother buys this 340 challenger as his daily driver. He does a little work on it, drives it for a while and it starts backfiring through the exhaust. He goes through the ignition, the wiring, the fuel system, timing chain, takes off the heads and has them redone, puts em back on, same popping. What gives??? Well, the one thing he didn't check or change again was the spark plugs. Turns out one of the insulators was cracked, not even bad enough to see, but apparently it fired erratically enough to keep it popping through the exhaust. With fairly new plugs or not, an insulator can get cracked just changing the plugs. Check it out, it's cheap enough to rule out, unless you go about it like my Bro, LOL!!!