Aftermarket exhaust has no 02 port, the pigtail and sensor are missing. Will get this fixed when vehicle is road worthy but until then how well can I expect this engine to run without sensor? Only has one code 21 - heated oxygen sensor signal.

 

The computer gets no signal from the o2 sensor it will run in default mode which is rich.
Short term not anything real bad [short trip to the store], long term you will get bad mileage, engine gets all carbon'd up, cat runs real hot and fails, exhaust will be hotter than normal [glowing manifolds], head gasket failures, etc. Lots of bad things long term are possible.
That little sensor does more than most people think.

 

Good to know.
Can I still set timing?

 

Lack of an o2 sensor should not affect the timing or vice versa. Someone else with more knowledge of the v6 computer may chime in.
In default mode the timing "may" be affected but I just don't know.