8" wheels what offset should work?

Problem I have found when talking back spacing is you have to clarify with manufacture how they measure. I think most measure to tire bead but some measure to strait edge on back of wheels the difference is about 3/8" in most cases.

Back to your question I think both will work however if your Dart does not have the low fenders like my 68 I would go with a 4" back spacing for best fit. The 4.5" backspacing is really close to leaf springs and may require a spacer to keep the tire from rubbing depending on the tire size.