"and the unit turns 360 degrees while the nut is stationary… "
thats because the nut works two ways and your only getting the nut connected fully to the tank and not to the petcock. try screwing the nut into the petcock a bit before you put the unit on the tank. this might ensure that the petcock fits better. as for the leak you should try putting alot of teflon tape around all the threads. that should prevent from leaking.
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^petcock ^bolt ^gas tank
see how the petcock can still go in further but the bolt cant because its already all the way in the tank screw? well you should prescrew the bolt onto the petcock thread just about right so that both the tank and petcock are at the ends of turning at the same time. it takes some fiddling but its all worthit. or if you can turn your straw without breaking it then you might be able to turn the petcock