I agree with David in that the pump have some added stress trying to pull from a 5/16 fitting. Another good option would be a universal tank retrofit kit such as this one from Aeromotive. . For the sake of saving money I might recommend installing a Walbro 255 on the 5/16 tank noutlet and see how it works before you spend the big bucks on a retrofit kit.
I havent had a stock tank for years so I am not sure what size the drain plug is however I would imagine anything 3/8" or over should be sufficient.