Engine, Transmission and related drivetrain.
With any transmission swap check that 1,3,5 or 2,4 do not rub the hole in the floor. Always keep in mind the engine and transmission does move rotationally from engine torque, slight forward and back from acceleration/braking and side to side from cornering. Not all is the fault of the rubber mount as this is not the mount or the transmission and in some cases not the stock engine either for this car but from some other vehicle. Too firm is worse than too soft. The whole point of using rubber is to provide vibration isolation. It's up to you to make sure it works. Any movement of the engine also moves the attached exhaust pipe as well, so give it a huge tug in all directions and all the way to the end of the tail pipe. All exhausts will be different and a mish mash of pipes and 'mufflers' so by definition they have been made to fit as well as possible and under load may rub, anywhere.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs