7.25 to 8.75 swap Dodge dart 1965

There are alternatives. (You will need to move the spring pads, and get 8 3/4 shock plates and u-bolts).
If the 8 3/4 is six inches wider, you will need three inches more backspace per wheel than you have now, to get the wheel/tire in the same location they are now. Somewhere around six inch backspace. You will probably need an aftermarket wheel designed for a late model mustang to get the backspace, they have the same bolt pattern as your 8 3/4. (Late mopars DO NOT. Close, but no cigar)
To get a matching front, you will likely have to have a 5 on 4 1/2 conversion, and a BIG spacer. A 5 on 4, to 5 on 4 1/2 adapter might work if thick enough, good luck finding one.
Bottom line? I think you would be happiest narrowing the rear.