I'm not much of an expert on tire sizes and fitment, but a 305 with a lowered rear end might cause you some rubbing issues. Try our search engine on the site. There are a lot of threads associated with tire fitment and what tires works best for your vehicle.

 

Here's a recent thread that might help. @Catmonkey is the man on tire and wheel fitment.

2007 - 2014 - fitment question: will tires on ‘11 SVTPP...

I recently installed an Eibach Pro Kit spring set on my 2008 Gt500, after an alignment, the car looks and rides fantastic. Last week got a killer deal on a set of 2011 Gt500 stock wheels which I really love the looks of. If I wrap the 11 wheels in the 19/20 stock tire size would I have any...

www.fordgt500.com

 

I have had the Eibach pro kit on 2 of my GT500, (both had 20's though)
My Black vert had 305-35-20's on the rear, no issues.
I looked at 19" rims quite a while ago & went with 20's as there was more tire choices (thing maybe different now)

A few pictures, you can see the gap & fender/tire
Colin