I have found a very good deal on a th400. What will be required to install in my 70? What donor cars may have a th 400 crossmember that will make this a bolt in? Drive shaft excepted.

Your crossmember should work. That's why the crossmember trans mounting pad has several holes.

Assuming you have a TH350, Tommy's right, your crossmember should work, but you'll have to remount it farther back and you'll need a new transmission mount. 4th generation's have the holes in the frame for either trans, if yours doesn't, you'll just have to drill some.