I got some good advice from a member on here. He said if your running a flex band then use the 3.8 ratio, if a solid band the 4.2 ratio.
As for the kit, this one is highly recommended here on the site.
Transtar Super Deluxe Transmission Kit For Chrysler TF8 (A727) 62-Up from Northern Auto Parts
Dont over pay for parts thru the big name companies. Shop around, you will find out you can get better deals for the same stuff else where.