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Your '78 has coils and control arms as does the Grand Ma. What are the chances all that stuff lines up between a '78 and an '87?
While the 8.8 may have discs and might be marginally more efficient due to a slightly different ring and pinion engagement, as per the prior post, you can get just about anything you want for a 9 ratio wise, disc wise, axle, etc etc. And it all bolts right in....