97hb

I drove to the muffler shop to have them weld my cat on. When I got there they said they couldn't do it because you can't use a used cat. The cat is from the car that the engine was pulled. Also when driving to the muffler shop I couldn't get above 1,500 rpm in any gear so I was going slow. After 1,500 it would die like its not getting fuel. I have an airfuel guage and it read lean above 1500 but fine between 1000 and 1500. Is this because I have no backpressure? All the exhaust is coming out the headers. Only code its showing is 65 (2nd 02 sensor). Im just worried about why its not going over 1500 rpm. Also anyone in Dallas know where to get the cat hooked up. Help me out fellas.