This is the only axle code listing I have saved but it only goes up to '67.
http://www.67-72chevytrucks.com/vboa...d.php?t=327764
Being 6 lug it could be a '63-'70 2wd or a '71-'87 out of a 4x4. IMO the easiest thing might be to measure your WMS to WMS and then you will know what length axles you need. All '63 to early '70 12 bolts have a WMS to WMS of 62". So you would need axles that are 30.5" long.