Distributor shaft length

There are different lengths between SB, B, and RB, but not between small block. Look which way the advance sits......B and RB rotate the opposite direction

You can also twist the rotor. A small block (rotates clockwise) will "spring" clockwise, then snap back.

Any year smallblock distributor will fit any other year. Be sure it has a vacuum advance. "Lean burn" had no advance, at least some had twin pickup coils. You don't want them. Be sure it's not an "EFI" distributor, which has a great big half moon shaped "deal" inside. You want an 8 point reluctor, single pickup, and vacuum advance.

ANY factory distributor you get will need to be re-curved for performance, as any of them came with a "smog" advance curve.