I don't know if they would fit, but the internal dimentions of the stock one are
Length = 241mm
Width = 92mm
Thickness = 53mm
Hopefully this is of some help

The sample filter is not a 'Genuine' part but a Japanese replacement made by SP [Part No. SP - 674] & it also carries the Datsun part number. [16546 - H2300] They fitted my GX airbox just fine, so I presume that they are exactly the same as the originals.
The Unifilter has smaller internal dimentions simply because of the soft foam construction, but this foam will cause no real problem if you stick with the dimentions above.