Wheel width question

Will depend on the rim offset. If they are stock wheels, the 7" Is the correct rim for that size tire. If the 8" has a more positive offset, you can use that one. 8" rims and wider usually have a more negative or reverse offset which pushes the tire to the outside of the fender opening.
The 7" wheel has 3.76" backspacing w/ -6mm offset and the 8" wheel has 4.5" backspacing w/ 0 offset. The car is a '73 Duster.