But... do you really want the hassle of 2 cars, 2 lots of rego?
I don't think putting an RB20 into a 260z is going to butcher it. It depends on how you do the install really, create a good loom, get your mounts fabricated (as far as I understand the dimensions are not that different, gearbox hole should line up no?)
Original is good, but RB20 should be a relatively straight forward swap and you can always convert back later if you really want to.