The 69 A12’s came with LY Rods. (The A12’s also had standard 440 nylon cam timing gears in case you were wondering). You can use the heavier 70 sixpack rods but you have to add a lot of weight to balance the crank and reciprocating mass. The LY rods are fine unless you are building a monster motor. You would be better off buying lighter and stronger aftermarket rods than going with the heavier 70 rods.
You know, I've had "big rod" & "small rod" cranks side by side & they sure look the same. I "thought" all the extra weight for balancing the big rods was in the harmonic balancer + flywheel/torque converter???
