Backfiring, loss of power

I have a 1991 - 34' motorhome with F-450 super duty 7.5l. I had it into a mechanic for some transmission hose repair because I thought that was the problem. After getting it fixed, on the way home I learned the catalytic converter was also gone. Neighbor cleaned the guts out of the catalytic converter (it has two large connections for intake and exhaust and two other connections with pipes attached, not sure what for.) And it seemed to run fine. Drove it 50 km and it started to backfire while decelerating, then lost all power as though the catalytic converter was still plugged. Most people tell me that the catalytic converter is not the problem, others tell me that the catalytic converter on this size needs the back pressure (perhaps for those other two connections) and cannot have the guts removed. Also but may not be connected, the automatic overdrive no longer works. Any ideas?