The worst one will be emissions if they apply in your area. A 96 is obdII and the ref's computer has to connect to the nissan computer. With a 90 powertrain that will take some work since the 96 had new style injectors, 2 more 02 sensors, and no vtc. If you convert to an earlier year harness and ecu like some have, it will fail emissions but the swap would be easier.
KongZTT GT28RS MD "I was very impressed with his mother mounts"