Will the rear axle fit?

The c-c measurement is center-to-center on the spring perches. No matter what you'll have to move the spring perches to match the A-body spring location, which puts the spring perches at 43". Unless you move the spring even further with a 1/2" spring offset.

You can use 15" rims still, I ran my Duster with 15x7", 4.25" backspace cop rims and 225/60/15's after I did the swap. But there wasn't any extra clearance, that's all I could run. The problem is you won't find a lot of positive offset 15" rims. And if you wanted to run wider tires you'd need a pretty significant positive offset. You can get 275's on a Dart Sport or Duster even with the stock spring locations. But to do that with a '68 B body rear you'd need like 6" to 6.5" of backspace.