74 duster backspacing

Sure looks like an 8.25.
255/60R15 with 15X8 definitely fits in the back quite nicely.
4 inch backspace works, these dusters/ demons / dart sports have alot of room in the rear well.
Here's my old pic with 255/60R15 on 15X8 Visions with 4 inch backspace. The fronts had 215/60R15 with 15X7 & 4.25 in backspace as well.
I also currently have a 8.25 rear axle.
20181010_181434.jpg

She now has 17X8 with 4.5 inch backspace & 255/50R17 tires in the rear. The front has 17X7 4.25 inch backspace with 225/50R17 tires.
Rear overall diameter is 27 inches, front is about 25.9 inches.
I am planning on doing 17X9's rear in the future after doing a rear disk brake 8.75 swap.
This is not to everyone's tastes, but the tires are so much better especially paired with aftermarket suspension.
My Duster.jpg