to test how well fuel injectors are working you can run an injector balance test... you will need a fuel pressure gauge and an injector pulse simulator.... i believe both can be rented from most auto parts stores.... the simulator plugs into the injector you pressurize your fuel system and then turn off the fuel pump.... most GMs have a fuel pump test lead in the engine compartment... so you just have to jump power to it.... run the simulator for a given amount of time on each injector.... and record your findings with the fuel pressure drop that you see on the gauge....between each injector there should be no other than 10 milipascals difference...... if one injector seems to be dumping less fuel then the others, then it is clogged or has a weak coil... injectors can be cleaner properly without replacing by cleaning the screen on it and blasting pressure through the nozzle.... the only way an injector truly goes bad is if the coil inside of it begins to wear out or if the plunger that the coil lifts breaks(which is very uncommon)..when it drops more than not it is cause by the plunger not seating properly, which more than not is debri getting cause in the plunger.. a lot of people replace injectors when they can repair them for MUCH cheaper, they just dont have the knowledge of how to do it.... btw your TBI unit will only have 2 injectors gm v8s have 2 injectors on TBI and everything else only has a single injector.... the TBI isn't a very efficient way of fuel delivery as i'm sure you can tell with 2 injectors powering 8 cylinders.... they tend to wear the coils out faster.... which is why most vehicles run SFI which is very efficient.... a single injector per cylinder and it only shoots once per intake stroke.... the MFI systems would spray fuel ontop of the intake valve before it opened and then spray again when it opened... the puddled fuel wouldn't be atomized well and would be partially wasted....

i know i explained more than you need... but its always helpful to have knowledge that may come in useful another day...

now for O2 sensors getting carboned up... do you have one before and after the cat or just one before... if you've got one after the cat and its getting carboned up then your cat is not functioning... the one before can be cause by rich mixture which can be cause by numerous things.... but a carboned up O2 sensor should actually cause a lean mixture, it will keep telling the computer that its not seeing much oxygen and so the computer will compensate by dropping how much fuel its using... which would then give you a lean condition and possibly a lean misfire.... which makes me think you need to replace the O2 sensor and also fix what is causing your rich condition that causes your lean condition