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Don't even THINK about "cutting a blackwall" - blackwalls haven't had a white underlayer for decades. Diamond Back will make you any whitewall width you want, on brand-new radial tires - that's their only business.
I should have been clearer there are Internet instructions for cutting existing whitewalls to a wider width - evidently thin whitewall manufacturers put extra white material under the outer black tire material. There is also wide whitewall 'paint' and of course, the old Portawalls. Crazy white rubber discs that go around the outer rim of the wheel up against the blackwall to LOOK like wide whitewalls.
The lengths people will go to is amazing-- I don't recommend any of these approaches...