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Truck cat convts depended on GVW. All trucks over a certain GVW were exempt. The GVW exemption was raised thru the years till all trucks used them. It was the main reason for the D150 model with it's increased GVW over a D100. Ford and Chevy also had versions of the 'heavy half ton'. 1979 had a change since a 78 D150 didn't have a cc and a 79 D150 did. In 1985, a D350 was still exempt.

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Most Mopar passenger car apps began in 1975. Trucks came in a few years later (lighter duty trucks first, heavy duty trucks phased in later). IIRC, the '78 Little Red Express was the fastest vehicle in USA, beating the '78 Corvette, in part because the Lil Red still had full dual exhaust + no cats and the Vette was more completely neutered. But I think '79 Lil Reds got cats too?