When you loosen the bolts, the distributor can rotate. Rotate the distributor clockwise to retard the timing. You will also need to short out the service connector by the ECU. Here's a great link for you:
http://www.c-speedracing.com/howto/timing/timing.php
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
Team B.O.B.® - Ballaz on a Budget