1990 4x4 full sixw blazer

Me and son have a 1990 4x4 full size blazer. here is the problem. We took the exhust system off the blazer and placed headers and put a flowmaster system on it. but the headers did not have a o2 sensor so we had to place on on the header. so it was running fine the the catco burned out. so we changed the o2 and placed a new one. then the blazer started to run with miss in it. and the service light came on and it was the knock sensor. So we changed that and put it on a tester and evrything came out ok. then the service light came came back on, and it has a miss in. and there is no power the most mph it will go is 45, then we just found out the tranny has some metal shavings in it. but it shifts fine still. we also changed the fuel filter. any ideas.