I have that same setup with Cragar SS wheels. The fronts are 4 1/8 backspacing and the backs are 4 1/4 I think. They work well for me. By the way you may be confusing offset and backspacing. Backspace is the difference between the inside edge of the rim and the mounting surface. Offset is the difference between the mounting surface and the wheel centerline.
So on my 67 I have 15x7s with 3mm offset and 225-60 tires in the front. 15X8 with -6mm offset and 245-60s in the back. Disc brake kit in the front pushes the wheels out a little so I could have used a little more backspace there for looks but I don't know how much room I had. Everybody likes a different look, but these work for me. Hope you find the perfect setup for you.