how to get 340 into 12,s

hi everyone ,had the car dynoed and it made about 340hp at the rear wheels,the same as onother car my brothers got which is a holden with a big block chev,the same wieght as the challenger yet it runs 12.1 at 108 ,we,re changing the springs in the msd as the ones it came with are the heaviest ones you can get ,we believe this is part of the problem and it should help wake the car up off the line ,this car flies once up and running,looked at the msd graphs on the different springs and it shows no power at all until well into the power curve ,so hopefully these lighter springs will bring it straight away