I think the 351w with a good set of heads will move the car nicely.
The AOD is bigger mainly in the oil pan area. The tranny holds more oil then most other trannies (ford), due to the OD. The reason the long tube headers give you so much problems, the collectors come right down to the oil pan area, and tend to get in the way of the tranny linkage. I got mine to clear, but it took a lot of torching, big bars...and the end result is a header collector about 2 inches off the ground and still so close to the linkage it makes it real fun to work on hooking the linkage up.
You are going to need the TV linkage, and another linkage to hook ur shifter up to the tranny. When I did the swap 4 years ago, there was nothing out there I could find. I know it is offered for 65 era mustang, but I would assume it should be for a 70. I used a B&M mega shifter, nice shifter, but lost the old look in the car.
The drive shaft will need to be shortened most likely (aod is longer), and they recommend using a t-5 yoke since it is stronger than others.
You can make ur own cross memeber, or by a nice custom made one.
My advice, go to
www.windsorfox.com, they have all the pieces I just said, and they are worth all the money to keep from suffering a lot of the headaches I did when I first did the swap. They also have a fairly detailed explanation as to what exactly you will need to complete the swap. I f you buy one part, buy the TV cable ($90) they offer, makes the tranny shift exactly liek the tranny was designed to be in the car--AND it is The KEY piece to making the AOD last.
Ryan