You cannot move the inside of the wheel wells very much, for the relocation will affect the sub-member for the spring mounts. Without surgery, it is easier to go to the outside for additional tire width.
Are you doing the quarters too? The issue with the outer wells is that they are welded to a bracing frame that is part of the roof structure so access to that spot is virtually impossibe with the quarters on. Otherwise it's a matter of drilling the spot welds and plug welding the wheelwell back on. How is the trunk floor and rockers? If it's an Ontario car then there must be more rot in that vicinity. Ask me how I know!! Stu
PM me with your email and I can send you a pic of the structure on my '67. I did outers on mine but my car is a bit of a retro mod not a resto and has an 1988 rear end on it.
Or I'm on the http://www.cmoc.ca/forums/ as CaptVirgilHilts too which is a Canadian Mustang group. I can post a pic there. There is a vintage forum there too. Where in TO are you? Stu