rear end noise and eating ignition switches

I have a 1970 240Z that has gone through two ignition switches in the past year. The second one was not genuine Nissan so I don't know if that had anything to do with it. One local mechanic said I had a hot wire grounded. However, that cannot possibly be the case(I don't think) because the battery doesn't discharge. Also, there is a terrible noise coming from the rear end that increases with engine speed and it sounds so bad that it gives the impression it is about to go at any moment. I thought for sure it must not have any oil in it so I checked it and sure enough, it had the recommended amount in it. Therefore, I am baffled once again as to why it makes such loud and terrible noise. Also, when I am cruising in top gear, the engine turns over at 3,000 RPM even though I am only going 55 mph. So, I want a different transmission that has an extra gear but, I don't think a 280z or zx transmission has the same ratios in the lower gears that mine does so, do I have to buy a brand new one from Motorsorts if I want overdrive with the same low 1st, 2nd, and 3rd ratios or Is it possible to find one in a junkyard in a 280 that has what I need.

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