It's really not hard to do a HG on a z31. You might want to look into the cost and labor of simply replacing the gasket, and sticking with the engine you have. I dont see it taking more time or money than swapping that Maxima VG30E. Keep in mind the work required in messing with that auto trans, exhaust manifolds (always rusted stuck) and swapping all your z31 VG30E parts over to the Maxima VG30E. Unless something is wrong with your VG30E i say use it.
Build up the maxima engine on the side with the require z31 parts, fresh paint, new gaskets, water pump, timing belt/tensioner, cams, a turbo, whatever else and swap in reliable performance in the future.
Just my .02
-Mike