1 7/8 headers on 69 dart?

The rest of your combo can affect the optimum header size but, in most cases, I believe that 1 3/4" or 1 5/8"-1 3/4" step would Work better than 1 7/8". Just because you have better flowing heads doesn't guarantee that bigger headers will provide more performance.

As for fit, TTI are supposed to be good fitting headers. I was very happy with the TTI stepped on my 360 LAX head Dart Sport. But little things like factory production tolerances, tweaks in the frame/chassis over the years and/or motor mount design/wear can affect clearances. I don't know if there is any difference in the exhaust on your heads.