ANY rear later than 67 that you use to replace the 67 8" will require the drive shaft to be shortened about 1", so measure carefully, the nose sticks out more on the stronger rear ends.
On my 69 when I changed to a 12 bolt I did not have to shorten my driveshaft, just change the U-Joint. When I changed the 12 bolt to the 8.5" 10 bolt I had to shorten the driveshaft by 1 inch
