Gregg's valve body won't work with the 4R70W. If you want a complete valve body replacement (this really is the best way to go), you can get a
LENTECH or
Performance Automatic. Both are very highly rated. You can find reviews at the Mustang forums. If you do any modifications to your vavle body with a shift kit, you are stuck with the results. If you replace the entire valve body, it's completely reversible and it can always be removed and placed in another transmission or sold.
TransGo seems to be more popular than Baumann.