Okay, so I have a bit of time to update.
Exhaust:
The car was running poorly once it got warm on the way to and from the exhaust shop. Missfiring at idle and running rough, although once going it seemed okay.
What I found once I got the car back to work was the bank 2 intake cam (front bank) was stuck on 40 deg advanced. Unplugging the solenoid controlling oil flow to the cam sprocket had no effect, so I quickly swapped solenoids with one off a blown engine. Success! I suspect possibly the solenoid got tweaked in the collision just enough to work okay when cold and seize up when it got hot. Really glad I'm not pulling the timing covers off again.
Runs much much better now, but still not 100%. The front bank O2 readings are a bit off, so I replaced it with a junkyard part from an Altima. I was using one from an older Maxima (the one from the donor car got smushed in the wreck), not sure if they're the same part number or just use the same connector so I grabbed one from the proper car to eliminate that variable.
The MAF readings are also on the low side of spec, so I popped in my spare test unit but it has pretty much the same readings. Going to check for vacuum leaks next.
I do have an exhaust leak on the front bank, goes tiff tiff tiff on startup and when you blip the throttle so I'll have to dig into that. Either the cheapo header has cracked already or the OEM gasket didn't seal somehow (warped flange?).
The gas in the tank is about 3 years old now so that's another variable I have to consider. I ran the car on one of those 3M injector cleaner thingers, not sure it ran much better on the alternate fuel source. It was quicker to pull the backseat and unplug the fuel pump than try to figure out which relay is the fuel pump relay (stupid setup on these cars, it's under the drivers side dash somewhere and unmarked). While I was there I decided to swap the Sentra fuel pump with the Altima unit. Not sure if the Sentra pump is up to the task and I saved the pump from the Altima so why not.
Freed from the Altima canister, compared to the Sentra pump, and back into the Sentra canister.
And the last new issue I'm fixing is the cooling system again. The car was collapsing the rad hoses when it cooled down. Looks like the issue is the el-cheapo eBay rad. The filler neck is tiny, smaller than the vacuum valve in the cap. So the vacuum valve isn't capable of opening to let coolant from the overflow back into the rad. I've been looking for a cap with a smaller vacuum valve that will fit but no luck so far. I've ordered a Vibrant weld on filler neck with matching cap, so if I don't find a cap that works I'll get the fab shop to weld that on for me.
I tried to cut down the valve on the ebay cap but can't get it small enough, plus I nicked the primary seal so it's garbage....