Yes, it does make a difference with the Thunderbirds. My OEM 'bird wheels had their mounting face 1/2 inch outward from centerline (inside edge pulled toward the knuckle). The tires were 205/75/15's. I checked the clearance at the upper ball joint - and then chose a spacer to maintain that clearance with the new wheel and tire combo. So it turned out to be a 1-3/4 thick spacer/adapter would work. The wheel centerline went from being 1/2 inch inboard of the mounting surface - to 1/4 inch outboard. The 225 tire had plenty of room in the wheel house on the outside.
The real fun on the Birds - dunno about the sedans - is at the rear. The closest clearance inside turned out to be from the leaf spring to the sidewall - behind the axle (the springs are not mounted parallel - wider to rear). At the outside fender lip it was at the centerline of the axle (when I sat on the fender to collapse the spring a bit). So at the rear I used a 1-1/4 thick adapter - and could have used a 1 inch to pull it in a little more - but the 1 inch thick adapter looked spooky to me (not enough meat for the studs).
I looked the tire wheel calculator - but it only had clearance specs for T-birds and trucks - no sedans. Guys have told me that if you want to have the same tire front and rear - even on the sedans - you are best to use 225's as the maximum and go from there. If you were to put the 225's on the 8 inch rims the section width would go out about .2 - to about 9 inches - pretty much even with the rims.
Steve Metzger Tucson, Arizona