need help! please

[mudchaser]

ok so here it is it is a 1997 gmc k1500 sle with the 5.7 a 6 inch rcd lift duel exhaust k&n air intake
all that was done when i bought it with a bad trany
replaced: 460le tranny spider injectors with multiport upgrade, spark plugs cap rotor and wires and egr
it ran great for a few weeks then the tranny started sliping and motor would also bog down (like a cloged cat maybe) so i pulled the codes and got p0307(7cylinder misfire which was the reason why i replaced the injectors) p1870 trany slippage p0154 and p0155 which were 02 sensors on bank 2 and p0420 a bad cat reading
what i thought change 02 sensors and bad cat reading wou go away since it get the cat reading by comparing 02 sensors?
so i change the sensors and no different truck still runs the same except no codes have came back on?
i have a feeling its a computer problem because of the new rebuilt trany is slipping one interesting thing the light for my gear indicator on the dash hasnt ever worked?
i tried to take it back to the trany shop and he says i need to fix the engine problem first. im lost and dont know where to go from here

[speedygonzales]

Is the misfire gone?

[bighaas79]

Comparing the waveforms from the upstream and down stream O2 sensors should show a failing cat. You could also try a back pressure test. The 420 code you had would be the same bank your misfire was on. If it was driven long enough with the misfire you could have easily toasted the cat. I would not be surprised if the cat was junk, however I don't believe the tranny slippage code would have anything to do with the rest of your codes, and there may still be something wrong with the trans. It is possible I suppose that maybe your tranny rebuilder didn't clear codes?