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

There isn't much in a 5" backspace. You'll have to widen your search to include 5" up to about 5.6", and plan on running a spacer in the back.

These are the rims I run on my challenger, but in a 17x8 and 5.25" backspace. With an A-body 8 3/4 and BBP axles a 5.25" backspace may clear on some cars with stock springs, or maybe a small spacer.

https://m.summitracing.com/parts/vsw-141h7865gm19