63 Dodge Dart, now let me see;)
You now have 3.75 back space with 7" width rim.
Going to 4.5" backspace will give 0.75" (3/4") more back space with a 7" wide rim. This will pull the wheel in 3/4" to gain more clearance at the wheel opening.
Now if you go to 8" wide rim with 4.5" BS you may only be moving the wheel away from the wheel opening by 1/4".
Going from 7" rim to 8" rim you are splitting the extra inch to 1/2" outside and 1/2" inside, only moving the outer wheel face in by 1/4" by using 8" x 4.5 BS rim.
This is where it gets complicated as to what the Front Spacing is by increasing the rim width to 8".
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Looks like you have plenty of room on the inside tire face to the leaf spring to do the increase to 4.5" back spacing.
☆☆☆☆☆
You now have 3.75 back space with 7" width rim.
Going to 4.5" backspace will give 0.75" (3/4") more back space with a 7" wide rim. This will pull the wheel in 3/4" to gain more clearance at the wheel opening.
Now if you go to 8" wide rim with 4.5" BS you may only be moving the wheel away from the wheel opening by 1/4".
Going from 7" rim to 8" rim you are splitting the extra inch to 1/2" outside and 1/2" inside, only moving the outer wheel face in by 1/4" by using 8" x 4.5 BS rim.
This is where it gets complicated as to what the Front Spacing is by increasing the rim width to 8".
__________
Looks like you have plenty of room on the inside tire face to the leaf spring to do the increase to 4.5" back spacing.
☆☆☆☆☆