904 kickdown

I have a 66 Dodge Dart with the original slant six, 904 transmission. I converted it to a "Super 6" 2 barrel, with the appropriate linkage. I have adjusted the linkage several times, but am still having problems. The transmission slips when cold between 1st and 2nd. Once it warms up, it shifts fine, but at no point do I have any downshift. It will lug the engine in 3rd gear unless I manually downshift it. There is also no passing gear. My instinct is that it is the linkage, but as I mentioned I have tried adjusting it many times and can't solve the problem. Is there a band adjustment that would help? Thanks for any input.