12 bolt length

[rjs53]

I have the narrow rear in my 69. I also have another 12 bolt rear that I traded a buddy for some seats. He said it was out of a 77 4x4 but not sure. Do I measure from wheel mount to wheel mount or axles? Both are 6 lug, and I don't want order the wrong axle. #3896839N GM61 are all I can find. Ring gear is 242. Measuring from the back of the flange its 30.5 measuring from the front of the flange its 31.5.