Great project! Good call going 9" rather than 8". You will spend less $ on the 9" and end up with a stronger diff. Definitely consider a stout set of welded-in subframe connectors!

 

I'm looking at the torque arm suspension from TCI, subframe connectors are part of the kit. They're saying it'll be superior to a 4 link. I think it'll definitely make the car stiffer and shouldn't have any problem handling the horsepower of the Coyote.

 

I was looking at the Ford Specs here to get the gear ratio. Says it's 3:15 for an auto, but 3.55 is optional on a manual. Is this really all that big a difference? Should I bump to the 3:55 or just try to match the specs for the auto all the way?

I haven't had the pleasure of driving the new GT auto - does it have good range of power stock?

 

I would leave the gears where there are or maybe go to a 3.31 but not a 3.55. The gearing of the auto's is different to the manual, hence the different rear gears.