Which back space would be preferred for a 69 Chevelle?

Should I run the same back space front and back?

Or maybe 15x7 with 4.375 BS front and 15x8 with 5" BS rear?

Tires will be 245/60-15 front and 275/60-15 rear.

Run 15 x 8's w/ 4.5 BS all around on mine now. Fit is perfect. With 15x7's you will want 4" BS. These are considered neutral backspace - neither positive or negative offset, so the tire gets centered right over the face of the spindle or drum.

what size tire do you have on the rear on you 15x8's w/4.5 back spacing

275-60/15. Bu there is plenty of clearance for up to a 295-50/15 or may even clear a 285-70/15.