I own a 1973 240 Z. Few years back, I decided with my mechanic to replace the four-gear box with a five-gear box of a 260 Z, without changing the differential. I did that because there was a sound from the original gear box on the fourth gear Now, I think that this was a mistake, so I am wondering whether it would be better to leave the five gear box from the 260 Z and replace the differential with a 260 Z differential or, leave the 260 Z gear box with the 240Z differential as it is or finally, try to locate a 240 Z gear box and bring the car to its original state. Could anybody help me to locate a 240 Z gear box and generally speaking, give me an idea as to what to do?