Get the IRS pinion flange (Ford Raing makes it) and put that on your SRA when you build it. Use the stock driveshaft. Thats what I did and all is well.
Swap a 03-04 cobra pinon flange onto your solid and you can use the stock drive shaft......i swapped my machs solid rear into my cobra i swapped the flanges they bolted right on and everything worked perfectly!
I just swapped to a Precision 448 u-joint from O'reilly and you can keep the stock driveshaft and the u-joint will work with the GT/Mach pinion flange.