94toyotadlx

This all started when my truck overheated and warped the head,due to a faulty thermostat.It was towed to a garage and the head was removed and sent out to machine shop.The head was warped and shaved,valves redone,also new timing chain and cover were put on along with head gasket.
The truck runs great except at low idle it has a misfire. I replaced the distr. cap and rotor,wires and plugs(NGK). This did seem to help a little but the misfire is still there at low idle.
I also noticed that the catalytic converter is leaking at both ends.I was wondering when the head gasket blew would this affect the converter,(glog it) and would this cause the misfire due to backpressure.
Any help would be appreciated,just hate to take it back to the garage.