Because there’s a lot more to it than h just adding cats to the exhaust system. You’ll need to collect data on your present emissions output and then you’d have to establish a realistic target to reduce emissions to. Then you’ll get into an EGR circuit carburetor, unless you’re converting to a “real” EFI system with plenty of inputs and outputs to control that sort of thing. Although EGR isn’t required to use cats, it does help reduce the amount of CO they have to convert to CO2 by reducing fuel the engine needs to run, but it needs a metric crap ton of timing when the EGR is running. EGR also reduces NOX emissions by reducing combustion chamber temperatures. And then you’re probably going to looking at exhaust air injection to give the cats some oxygen to convert the CO to CO2. Then have everything sized properly for the catalytic converters to maintain operating temperature. I can’t imagine 3 inch dual high flow cats doing a whole lot of anything on a 360 as far as emissions reduction goes. The other question is, how stock or non stock is the engine and how performance oriented is it?