I think you will find that the 115/108 reference means they fit both 5 on 4.5 and 5 on 4.25 bolt pattern.
As far as backspacing, I don't know. they are .5 inches wider, with less offset so they will be farther out towards the fender lip.
They would be 5.74 backspace, 2.26 front spacing, were your current 7 inch wide ones are 1.825 frontspacing. Can you go with the wheel almost half an inch further out?
My 9 inch wide wheels have 6.295 backspacing, 3.205 front spacing, I have 245/45/17 tires and a supposed 1.6 inch drop, and I could drop it from the sky with the wheels turned and not have tire to fender contact.
97 Mark VIII LSC/Silver/Gray leather
97 Cougar 4.6 Sport/White Opalescent Tricoat Metallic /Gray leather down but not out?
89 Cougar LS 3.8/Oxford White/Gray velour/HG need replaced AGAIN
00 Grand Marquis LS/Vibrant White/Gray velour
63 Falcon 302/black and rust/shed isn't moving
82 F-150/Oxford White/dumb VV carb doesn't pass emissions