You 'can' swap a SR bell-housing onto a Zed box with a couple of small mods to the BH.
I really wouldn't bother though - SR boxes are plentiful & cheap enough.
Tuck your current manual box away after the conversion or sell it to fund the SR conversion.
FWIW, the early 240Z box is called a 71A (has the output flange at the back of the box).
Late model 240Z & 260Z is called a 71B box,
And the R31 skyline, R32 GTST Skyline & S13 boxes are all called 71C's.
They are all just evolutions of the same box.