The IRS uses a different pinion flange than the SRA so yeah, you need to swap over the SRA flange from your DS. For the yoke, you need the 27 tooth yoke for a T56. However you wind up with the 27 tooth yoke and the correct pinion flange for your rear, that is what you need.
For the crossmember, that rubber piece is the trans mount so yeah... thats needed.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.