The 4R70W and the 4R100 and not in the least bit alike, the biggest thing they have in common is the Ford Oval on the side.
The intermediate shaft is the weak link in a 4R70W, there is actually quite a lot you can do to one, I'm testing some now under conditions pretty similar to what you want to do, should be ready to sell by summer.
If you have a 4R100, we have a great program in place already. The Monster Box sells for $3120, free shipping to your shop or door, 2 year warranty that covers a new transmission, shipping and the labor to have it installed and all Monster Boxes ship with a Factory Tech Total Domination Converter (which alone sells for $950)
We also make really good valve body shift improvers for the 4R100 and the 4R70W as well as converters for both transmissions in both "budget performance" or "price be damned just the best darn converter made" price range.
Hope this helps, let me know if I can help you in any way.