Even just wrapping some wire around the thermocouple and the manifold should be good enough, you are just looking for consistant contact so you can compared the stock settings manifold thermocouple voltage and your new voltage once you lean the engine out. You don't really need to know what the actual temps are.
Anyways, don't wreck your car for a few mpg...
If your exhaust looks anything like this, you are probably in for big repairs!