Hot water tank temperature- pressure relief valve

The hot water tank temperature/pressure relief valve was leaking water, so I was trying to remove the valve and install a new one. I see there is some white plumbing tape around the threads and there must also be some rust. I assume the factory probably tightened the valve too much when it was installed. I tried to remove the valve with a crescent wrench and a pipe wrench and I can't budge it and in fact I broke of a piece of the spout trying to get the valve to turn. I don''t know what type of wrench will work, because there is limited space to get a wrench around the valve. Any thoughts on what might work? I sprayed some rust busting oil around the thread opening, but no luck so far.