Kim, 175's should be perfectly fine on the '71. I ran 185-70s on a '65 with only a minor rub at the rear during hard high speed cornering. The '65 now has 175's on it with no indication of rub at all.
Incidentally, the Sumitomo's seem to be a good choice for tires right now. I'm putting a set on a '72 shortly.
Jeff