The Mopar Performance part you got from Mancini's is designed to be used with the 4 bolt to 2 bolt adapter plate and a Mopar Performance aluminum master cylinder, that is why it is still to long even when it is adjusted all the way down (extra thickness of the plate and depth of newer master cylinder). I checked the Chrysler part number you need (3580119) and it is no longer available and I don't show any of the usual NOS or used parts places having it in stock.
You might try Master Power Brakes at http://www.mpbrakes.com they have a part number AD1303K that is an adapter plate and adjustable pushrod package for manual brakes. You probably don't need the adapter plate but it looks as though the pushrod has more thread on it then the Mopar part does so you can adjust it further, they might even sell the pushrod by itself.
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