235/75/15 or 30x9.50x15 are about the safe limit on a 7" wheel. Any wider and you will lose stability from lack of proper sidewall support. 32x11.50x15 tires, which would be best on a 9" wheel, are about the limit with stock hight suspension, clearance might be a bit tight in the rear. Best comprimize would be a 31x10.50x15, but you will need an 8" wide wheels. 8" wheels will be just fine, use one with a '73-'87 K10 or K5 Blazer fitment, 3-3.5" backspace.
