The rear raked look can easily be solved by a pair of weight jacker lower control arms. Plus if you have drag slicks for the rear you can also raise the back for clearance if you ever go to the track.
I had a slight rake on mine.
I bought the Steeda aluminum jackers and they are stout enough for a street car and ride quality is unchanged from a stock lca.
I can also tell the axle has become one with the chassis now ... it doesn't slop around.
If the Steeda weight jackers are too spendy for you, then you should buy the StangNet (Pro3i) units from the parts section ... I heard they work really well also.
As for springs you could go with either, since you only want a 1.25-1.50" drop. The Eibach Pro-Kit is a softer spring than the Steeda.
Here's a link to spring rates ... remember too stiff of a spring puts more stress on the dampeners (shocks/struts) and wears them quicker. You definitely want to match your dampeners to your springs for best ride quality.
Ps - Factory springs are a progressive 425-530 up front.
http://www.miracerros.com/mustang/springs_calcs.htm
I also suggest you read the whole website ... if you haven't already.
Personally I think the Progress Tech springs are the best compromise.
http://www.miracerros.com/mustang/t_suspension.htm