Re: AC compressor
The 5.0s (both Chev and Olds) got R4s. The switch over happened in the late 70s, I believe. Searching rockauto for a couple of minutes will give the exact year...
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I'm back with more info. I think the problem here is that the car is an export car. Likely got different equipment than North American cars.
I have searched, and it looks like your car is supposed to have the R4 compressor. According to the info I have found, 1977 was the last year for the A6.
You can probably buy the compressor for the 77 and install it on your car, if you already have the A6 on there...