Correct me if I'm off a year or two.
71and back had one bolt hole that was not concentric.
72 + on the bolt holes were evenly spaced this is on the crankshaft pulley I'm talking about.
You can take a chainsaw chain file or a rat tail file an oblong one hole to make that pulley work.
If you look at the brand new aluminum pulley sets for Big Blocks and small box you will see they all have one hole that is oblong. That way the pulley fits all years of motors.
