I just went through this a few months back. Hopefully, you have done some research on the net to find and read several articles, and how to's so you know everything you need before you start.
Yes the crossmember sold by mustangs unlimited will fit without further modification. That's the easy part. Do you know about all the other parts you are going to need? In addition to the properly balanced flexplate, AOD block plate and inspection plate cover, longer transmission mounting bolts, C4 transmission mount, and either pre-formd transmission lines from Classic Tube or the skill to use adaptors and make your own lines, you also need: Lokar TV Cable, Lokar cable bracket, B&M aod lever kit, and an adaptor to hook the TV cable to the carb. If you have a Holley or Edelbrock carb, an adapter is available. You may have to cut your exhaust or modify it to fit with the AOD. I sure did. Let us know if you need more information.