http://www.energystar.gov/index.cfm?...p&pgw_code=WHC may have useful info.
On your leaky relief valve, did you dry lifting the lever fully to drain off a few gallons a few times, and then press down on the top of the valve?
Did you try a really big pipe wrench? The hard part is to keep the water heater from moving. Hmmm... somebody should make a crow's foot pipe wrench attachment for an impact wrench. If they did, it might cost as much as a water heater.