There really is no way of getting around the suspicion that people have for photographers now. Long lens or not, I think it's just a case of being forthright in your approach. Looking presentable and perhaps clean shaven helps too! Leave the trench coat at home. I have to say that the Callahan images have a unique intimacy that you don't often see in street photography. Have a look at Michael Wolf's 'Tokyo Compression' series too. The involved or 'caught in the middle of a circus' approach has been done to death, there's little left to say.
I looked at the Tokyo Compression and it's really refreshing to see. I also agree that using telephotos for street stuff is a nice way to have a different view. From what I see now this genre is too flooded with up close and in your face pictures.