I don't know about a 68, but I have a 67 Impala. Right now I'm running a 295/65R15 tire mounted on a 9" wide with 5" B.S. wheel. In order to fit that combo I had to modify the frame in that area. Very unlikely an approx. 3-1/2" B.S. will work. It will most likely push the tire/wheel way too far outboard.
Plus a 10.5" wide wheel is too wide for a 275/60R15 tire. Max recommended wheel width for that tire is 9.5".
Your best bet is most likely an 8" wide wheel with a 4.5" B.S.