I've driven on the street with both my 79 and 82 cars for 4 years with skinnies on front, no problem
In 79 and 80, base model Mustangs acutally came with some very tiny 13 in tires, before 14 inchers were standard.
So the typical 165sr 15 or whatever is not really that much of a big deal.
That said, they are not for corner carving at all, and Weld does not warranty the 3 1/2 in width for street driving. I bought the 5 in wide ones, and used the 165 tires.
3.5"'s are a coin flip on the street. if you live in the north where bad roads are the norm.. think twice about it. if bad roads are not an issue, go for it. but be conservative in your canyon carving