[Rulito]

Hi, first of all, excuse me my bad english.
Originally i had a 69 barracuda with 2 barrel factory intake and factory carb, points ignition, 4 speed trans and 3.2 gear on a danna 44, i went to the drag and the 1/4 mile was on 16.3 seconds. Then i change the intake to a Weiand X cellerator and a Carter 600 cfm carb, Crane Hi-6 Ignition, 3.73 gear, i welded the diferential in order to obtain traction from both wheels, i went to the drag again and i get 17.5 seconds!!!...
i have a factory cam, but today i received a new 274 comp cam and im planning to make a dual exahust system, so.. the question is.. why my car get slower than before?, what can i do in order to be faster.
Thanks for your atenttion