Definately not a good idea. The O2s needs to be fairly close to the motor. Must be before any catalyst. As a rule of thumb don't place it any farther back than the factory would. Here's why...any exhaust leak(even a minor one) will bias the O2s. The extra oxygen content will trick the O2s. Therefore you would get an improper reading. You can buy a O2 bung through Holley (available from Summit racing) and weld it into your headpipe. Then blug it while not in use with a Volvo drain plug.

What kind of systems are you wanting to tune? If you're trying to tune a carb setup it will be tough to use the O2 setup because of the narrow band of the sensor. Most carbs will be extremely rich unless you use a feedback type. I tried this with a Volvo I have and finally settled on an aftermarket fuel setup and a laptop computer. Now I get the total picture...fuel, timing, rev limiter, and decent mileage. It was expensive but prices are coming down all of the time. Good luck