For catalyst to work correctly the air/fuel ratio must be accurately controlled (the main reason that auto manufactures went to electronic fuel injection) and very close to stoichiometry which is probably a lot leaner than any engine calibration prior to the mid-70s. I suspect with a stock 235 a catalyst would become extremely hot as it burned all the excess hydrocarbons until it melted down and either plugged the exhaust or became a through pipe. I have seen exhaust systems run so hot that the steel is virtually transparent (well past red hot).
