If I install my FJO Wideband O2 sensor and splice it into the stock O2 wiring for the ECU, will it work with the stock ECU or will it only with the aftermarket one later on? Or will I need to run a completely new wire into the ECU from the FJO wideband?

 

There is absolutely no point to installing a wideband on the car unless you are running a programmable ecu. None. Spend your money on the right things, first.

Also, the FJO has a narrowband output that is meant for feeding the ecu the 0-1V signal it is looking for.

 

I think it's a reasonable question.
If you're a person who's done a couple of mods (not enough to go standalone ECU yet) it would be interesting to be able to replace the stock O2 sensor with a wideband (assuming you're going standalone in the future) whereby you could:
a) use a display/gauge to monitor AFR to see what the stock ECU is doing and how lean things might be getting
b) keep feeding the stock ECU the voltage it expects (as Rynberg points out)
It might be both educational and could help prevent an angine death. At the very least it might help in the decision to get that programmable ECU after N mods.

 

The stock ecu runs pig rich, even after several mods. If you want to test it, you can go to a dyno with a wideband for $60, instead of spending $500+ and a lot of effort. I don't see the value of a wideband with a non-programmable ecu.