Biggest tire I can put on factory 1968 Dart Rim

I won't even bother - not worth the hassle. I've started giving my opinion on such things by PM to avoid your lengthy tirades on the subject....

You do realize that your whole argument basically boils down to you just saying “I run the wrong size tires because I can’t tell the difference between good tire performance and bad tire performance”, right?

A properly fit tire performs better than one that isn’t. Not being able to tell the difference says more about you than anything else.