12 bolt length
 

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.

Re: 12 bolt length
 

Guess I'll answer my own question. Maybe this will help others, still don't know exact year of wider housing, but around 78 k10.length of wider axle compared to my shorter 69 axle the studs are a little longer on the wider rear, so it would be about 3/4" longer on each side