still trouble with 2-3 shift

I went in there today and changed the 5.0 kickdown to the 3.8. After 2 hours of crawing in trans fluid it does exactly the same thing ? NO different. I wonder if there is a problem with the valve body ? Once in 3rd it is fine. It stops the rear wheels dead for a split second then away they go moving again. The slower you are when you shift the harder it seems to lock up.I am starting to get pi$$ed off with this. The darn motor runs so good, I guess I had to have a problem somewhere. I guess next step is to change the valve body and see if that cures it. fastback 383.