Ed, I got the spec straight out of the 1969
Chevrolet Chassis Service Manual. I just double checked the spec in the 1968
Chevrolet Chassis Service Manual, it's the same = 60 ft. lbs. Ed, each of the five bolts also has a heavy duty lock washer on it. I guess if you are still concerned, you could put loctite on each bolt for extra insurance. Also when I have done these in the past, I have tightened them down in a star pattern (like a wheel) in four rounds:
1) 30 ft. lbs.
2) 45 ft. lbs.
3) 60 ft. lbs.
4) double check at 60 ft. lbs.
But that is just me.