Fast Ratio Pitman Arm...

RustyRatRod said

There are both spline sizes available. But, to my knowledge, no one makes a fast ratio idle arm for the early A.

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If it's any consolation, the AAR Cudas and T/A Challengers used the fast- ratio pitman with the standard idler. Not ideal, but was good enough for the factory.
If you're running headers, I can just about guarantee there'll be interference.