plugged exhaust?

I had a 87 Mustang 5.0 and a 93 F450 work truck with clogged cats. They both ran just like youre saying great at idle but wouldn't run over 45 mph and were getting really lousy fuel milage.

I would also consider the carb or fuel system as the problem since you didn't have any issues before changing it, But the cat would be next for sure. On that 93 work truck I had. It had blew a head gasket and the anti freeze in the exhaust system just destroyed the cat.