So I'm selling my catalytic converter and this guy called me asking if it will fit on his 99 with a 7.3 my truck was a 03 6.0 thanks every one

 

Drop in...doubt it. Weld it in...sure why not.

 

The 7.3 didn't even come with one from the factory did it?

He could make it fit, but it's not going to "bolt up" and I'm not sure there even the same diameter exhaust.

 

Your best bet would be to check with the state referee station. The People's Republic of Kalifornia has some real hokie smog laws about cats. It's as specific as to WHERE they are placed in the exhaust system. If y'all have never been there, it's something else to deal with.