switching 14" axels to 15"

The axles have nothing to do with the size rims/tires you can put on the car. A-body cars through 72 (and non disk brake \6 cars through 76) had a small 4" bolt circle for the wheels. You can get 15" wheels from the aftermarket that fit the 4" bolt circle but have limited choices. The more common thing to do is up grade to the more common 4.5" bolt circle then the wheel choices are limitless.

Stock 14" wheels whether they were 4" bolt circle or 4.5" bolt circle would be 5.5" wide and about the biggest tire you could fit on these rims is going to be a 225/60-14 tire. If you can find wider 14" rims the biggest you could fit under the front without interfering with the fenders is going to be a 235/60-14 and the largest 14" tire that is still readily available is a 245/60-14 which will fit under the rear with no issues.

If you go to a 15" rim with the proper width a 225/60-15 is about as bif as you can go on the front and a 255/60-15 or 275/50-15 will fit under the rear with no issues.

If you carefully select diameters, width and back spacing 16, 17 and even 18" rims with 50 series or lower aspect ratios will fit.

Your 73 Duster would not have come from the factory with the 8.75" rear end. If it was a 340 car or had the optional heavy duty rear end it would have been an 8.25 rear end. In either case the car would have the larger 4.5" bolt circle.

If the 8.75 in your car was swapped from an earlier A-body car it would have the smaller 4" bolt circle. Your choices are to; have the axles and brake drums redrilled, or, have new 4.5" bolt circle axles made with the B-body offset and you could use readily available B-body brake drums.