What u-joint do you want? Mopar car Dana's came 29 spline, are your gears from something else? 66/67 8.75 rears came with 10 spline yokes with large 1330 u-joint. The 1330 is the same as a 1350 or a 7290 except cup size. Not a great picture but you can see 10 spline.View attachment 1191182
MO 7260 u-joints never came on a mopar car Dana 60. The Dana 60 came with 1330 u-joint 66/67 and in 68 to 72 7290 u-joints. Any pictures, have you taken anything apart. Think your looking for the wrong yoke. All were 29 spline MO.