I am restoring a '71 Mach 1 and looking for some suggestions on Tire and Rim Sizes. I m thinking 15x7 (225/60/15) for the front and 15 x 8 (255/60/15) for the rear. Any opinions, thoughts on this matter would be greatly appreciated.
Personally I’d stick the (radial version of) the OEM sizes. The most I’d change is to put some classic 5-spoke mags on it…although I can’t remember ever seeing one without 5-spoke mags and wouldn’t be surprised if they came that way. That was a great looking car to begin with.
In short, I wouldn’t do anything to it that wasn’t available in '71. Today’s low profile tires do handle better, but mixing old styling with modern styling doesn’t usually work well.
Before you think about different tires sizes, have you done a figment test? The best place to ask these questions would be on a forum dedicated to these cars.
http://mustangforums.com/forum/classic-mustang-general-discussion/578734-71-mach-1-a.html
That’s pretty. Although the wing type spoiler option with the front chin spoiler and some Thrush sidepipes sure would add to the look! 
And we ALL got lettered sidewalls on our cars! Definitely…5-spoke mags with lettered sidewalls IMHO!
My vote is for retaining stock wheels and tires if the car is pretty much original as to paint color, engine, and so on.
according to tire rack, 215/65-15 front, 235/60-15 rear. The rim sizes sound about right, maybe a 15x7.5 on the rear but 15x8 would be OK. Just make sure the offset is correct for a rear wheel drive vehicle.
According to Tire Guides. the original tire size varied according to the exact model - but among those were: E78-14, E70-14, F70-14, and F60-15. Note these are Alpha-numeric sized. Needless to say, this sizing system has been replaced.
If you want to be totally authentic, Coker Tire has reproductions of the original tires. They are pricy, but they are authentic.
If not, then there is a whole world of options depending on what you want to accomplish. I’d recommend you spend some time at Tire Rack’s web site. They have a ton of wheels and tires and the options are almost endless.