1968 Dodge Dart Driveshaft

FlamesAreWicked said

That’s the U joint. The 7260 is for your lighter duty 225 or 318 models typically and the 7290 is for the more performance oriented models such as the 340 and 383

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Bingo. That's what I was assuming when I saw the numbers but wasn't sure. But if you look at the book again the 51.53" measurements are also A833 transmissions. The a904 only has one ujoint size listed 2760 and shows the same number as before 52.25."