I'd set it by ear. Loosen the bolt on your distributor put a mark on the manifold and base of distributor (just so you don't lose your current position) Take it out on a lonely stretch of road and tramp on it holding it through the upper end of your power range. If you hear an extended ping, retard your timing. Repeat until you hear a slight very short ping at the upper end of your power range. Tighten the bolt up and your done. If you prefer, put a timing light on it after just to see what degree of timing you are at and adjust with a light from there if necessary.
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91LX 5.0 ragtop, K&N, Mac Pullies, Centerforce, Full Dynomax, 3.27r
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