Well I installed underdrive pullies a few weeks ago and got a Optima Battery and now when I am driving at night my stereo is on, stock stereo and lights and fog lights are on and at a light or stop sign My lights and dash lights go dim untill I start again what size alternator should I get I am leaving the stereo system alone so what do you all think?

 

The mustang 5.0s will do that with normal size pullies, much less an underdrive pulley

Go with the 130 amp alt and no lower. You will be happy with that

 

I agree either a 130 or a 160. Are you planning on upgrading to an electric fan sometime?
Plan for the future.