What size 17 in rim will work on 68 Dart Converitble?

With a 7" wide rim and a 3 3/4" backspace you won't fit much of a tire at all. To run 225's you need at least a 4.25" backspace with a 7" rim and the 73+ disks.

Summit does let you filter your searches, you can filter by bolt pattern, wheel diameter, width, even backspace and offset.