I've run 215/65s and 205/75s on my 68. Both fit fine. The wheel I used was a stock 15" steelie- 5.5" wide if I remember correctly, but don't quote me on that.
Problem is that even a 205/75/15 isn't supposed to be mounted on anything narrower than a 5" wide rim. And the stock steelie's have a +6 offset, so, they have significantly more backspace than the OP's chosen rims and will be further inboard, which allows for a taller tire.
