I just wanted to know if a rearend that is a 7 !/4 fit going from a 79 Dodge Diplomat to a 77 Plymouth Volare. Does anything special need to be done?

Yep, direct swap. The ratio may be different, though, which would require changing the speedometer drive pinion and would affect the car's performance and mileage.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Both the 77 Plymouth Volare and the Dodge Diplomat have 318 cu. in motors. Would they both not have the same ratio? I have a 7 1/4 in my slant 6 77 Plymouth Volare. I own both a slant 6 and a 318 cu. in Volare. One is a two door and the other a four door Primiere. I hope they both have the same ratio.

Maybe, maybe not. Remember, these cars are approaching 3 decades old, so even if they started out with the same ratio, they don't necessarily have it now.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.