Sounds like your doing everything right. Sorry to sound like a jerk and doubt you, but I don't know you, and I the trans I did last week didn't have any kickdown linkage.
It sounds like you might have to trust your transmission builder? At least he says he will fix it for free?
I think if the slippage was just the converter then the clutches should look good. It is a good idea to have the converter cleaned/checked and rebuilt with all the problems your having.
On the otherhand, I with everyone elses opinion that the clutches are slipping, or there is a really bad fluid leak? You may want to pull the valve body and pressure check the clutches and servos with compressed air, just to see if you notice any really obvious air/pressure leaks.