I have a 2012 GT with the Brembo brake package. I want to get the FRPP strut, shock, "K" springs kit. American Muscles says I do not need a adjustable panhard bar but I need to get the GT 500 strut mounts. Late model Restoration says I do not need the GT 500 strut mounts but I do need adjustable panhard bar. FRPP tech line says it is designed to install and then go get front end alignment and that's all you need.

 

Im not so sure about the FRPP Struts. Never used them. BUT
When the 05-09 Mustangs came out the aftermarket struts were designed to fit them. Once the new bodystyle came along the struts would still fit but NOT WITH THE OEM Strut Mounts. So you had to buy GT500 Strut Mounts to make it work.
Also the OEM (GT) strut Mounts are notorious for falling apart and just being poor quality which is another reason to go with the GT500 mounts.
Fast forward and Koni has released (well, last year sometime) struts that do not require the 05-09 strut mounts. They will work on the 11-14 (don't know about the 15) WITH THE OEM mounts.
So if the FRPP fit 05-14 then you WILL need the GT500 mounts to put them on a 2012.
If they are specific (05-10 for one part number and 11-14 for another part number then you wont need the GT500)
The adjustable PHB is always a good idea. Is it 100% necessary? No.
But a good phb is a nice upgrade from stock and gives you the peace of mind to know that your axle is centered and it is a sturdier piece with less weight.
I hope the whole struts thing is understandable. It is kinda weird. Clear as mud..lol

 

You will need the strut mounts if you're running the frpp dampers. You might be able to get away with the stock mounts for awhile, but eventually they will start clunking. Best to do it right the first time. I would recommend getting an adj panhard bar as well. Your axle is going to be thrown off from the drop.