I'd say your assessment is spot on. With the 15x7's and 3.76" of backspace the absolute best you'll do is a 205. You might consider a 205/65/15, as that would be 25.5" tall. But even a 205/65/15 on that rim will be 1/16" closer to the fender than my 275/35/18's are, and my fenders are rolled and pushed. Granted, my car is substantially lowered and it's got a Dart fender not a Barracuda, so, there are differences and it might work for you.
Same for the 15x6's. A 225/60/15 is still going to put your tires 3/32" outside of my 275's. Maybe possible, but you're definitely in a spot where "fitting and trimming" will be required. A 215/60/15 on that wheel would be a much safer bet.
What you really need is a 15x7 with a 4.25" backspace or so.
In the back I don't think you'll have any issues with the 275's on 15x8's, but the backspace will depend on what width you make that 8.8.