I Just put 15x7 keystone klassics with 3.75" backspacing on the front of my 68 barracuda with 215/60-r15s and one side is catching the front edge of the fender. My car has been hit on that corner so I may be fine if I pull the fender forward a bit but then I might have a gap. The other side does clear but just barely. I'm glad I didn't get the 225/60r15s I thought I wanted.
You might be ok with the 4.5 inches of backspacing, pulling the tire in just a bit would help a bunch on my car.
I just finished the BBP conversion on my car as well.
