I agree,it should go faster. Tire slipis not the only problem. MPH is low. Were you running with the air claeners in place? That could be a big chunk of it. Beware of 40 degrees timing . It may be quicker but I've seen a few torched decks. For what its worth back in the day my Roadrunner would run 13.60@107 on 255 60 15 T/As. The very first pass with slicks was 12.32@114. Your combo will improve. My buddys 65 belvedere with a low rise dual 4,906 heads,557 Mopar cam runs 11.50 @115 all the time. I'll bet yours will be quicker.
Doug