largest rear tire on stock wheelwell 69 dart

The wheel makes little difference unless you are exceeding the measuring rim the manufacturer uses.

You can fit 9.65" section width. 10.5" with offset hangers.

The problem is.....I can only find literally one tire between 9.4" and 10.5" for a 14" wheel.

I'd be trying to squeeze this under there. https://www.summitracing.com/parts/mnh-mss002/overview/

It's going to be real close. If it didn't fit, you could put a set of offset shackles on it and they would fit for sure then.