Ali in the 70's did for a big part rely on his ability to absorb punishment, but he also relied on hugging, accuracy and his ring iq as well so its not the only factor. Pac doesnt rely on his chin either but its good enough for him not to worry about it when taking risks. Guys that rely on their chin are usually come foreward wearing you down type of guys like margarito and chavez.
Chavez Jr is the epitome of this. Not really much else to him, at all. I can see letting a guy punch himself out. But not on your chin!