First, would love to see some pics of that car
Pretty sure the trans and every thing rearward would have been left natural. Drive shaft should be bare steel, and there should be a couple of painted rings on it. Not sure what colours would be on the 76. The rear was most likely natural from the factory as well, keep in mind the centre diff, is cast and the axle tubes are steel.
On my 77 I used Eastwood paint. They have different colours of silver, for bare steel, cast grey, bare aluminum. In the end they look nice but they still look like paint compared to the real thing. On my current build I am taking or at least attempting to take everything back to bare steel and coating it with a product called 'shark hide'. Its a clear that is supposed to last, not yellow and be very durable. Parts are not supposed to rust with it applied. Time will tell.