Slow cuda 68 help plz

Hi, excuse me my bad english. Originally i had a 1969 std .318 2bbl carb, points system, stock intake, cam and heads and 3.2 gear on a danna 44. i ran 16.1 seg quarter mile. Then i installed a Weiand X cellerator Intake, Comp Cams 274 XE. MSD ignition. Elecotronic Distributor with lightweight springs, 3.73 gear. and the car ran 18.3 seg!!!!! almost two seconds slower.. why can happen thesE? any ideas? thanks