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