Wideband O2 install?
#2
it would be difficult, in my opinion, to get the proper angle of the sensor to prevent it from getting damaged. I've heard that water vapor from the exhaust (once cooled) will pool up in the sensor which will decrease its lifespan. It needs to be mounted in the 10 - 11 o'clock position approx. If you can somehow drill that hole in the proper place, something like this (https://www.summitracing.com/parts/hok-71014303rhkr) could be used to put it up there.
#5
its probably possible, but i wouldn't want to risk having a pinhole leak which would throw off your o2 readings. Probably depends on your welding skill level. I'm a self-taught welder and I wouldn't do a good job with it on the car.