8.75 narrowing in a 72 dart

Whatever width it needs to be to center your choice of wheels in the wheel well.

Seriously, your best bet is to figure out what wheels you want to run and what backspace they come in and work from those to determine the width. For 15” rims a narrowed rear axle works great. If you’re looking at 17’s, 18’s or larger a narrowed rear axle may actually be a hindrance.