Different drive shaft?

caferacerx said

also 7 1/4 uses 3 1/2 inch shock plates and U-bolts
8 1/4 and 8 3/4 use 4 inch shock plates and U-bolts

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7.25 uses 2.5" u-bolts and the 8.25 & 8.75 use 3" u-bolts.

FWIW, you can elongate the holes in the 7.25 shock plate 1/4" to the outside and use them on the 3" axle tube of the 8.25 or 8.75. I have been running this way for over 40K miles now.