Can catalytic converters be adden to pre 1975 cars WITHOUT AFFECTING PERFORMANCE?
There is such a thing as a high performance catalytic converter. I don't know if it is available for your year mustang, shop the inter-net. Older cars like yours were designed to use leaded fuels, no longer available. The right thing to do is to go to a machine shop and find out how much it costs to do a proper "unleaded fuel" valve job on your engine. Then you can use un-leaded fuel and not cause engine trouble or wear. If you want a Cat Conv. on your car, it is probably because your exhaust smells like rotten eggs. Adding a Catalytic Converter would probably stop this. De-sludge your engine as well, since there is probably a large amount of varnish built up inside. Years ago, lead was used as an additive to lubricate the valves, guides, seats, and seals. Un-leaded fuels do not lubricate quite the same way or burn at the same temperature. A little engine modification is usually called for.