The smallest tire GM put on the R10's in 1987 was 195/75/15, the largest was 235/75/15.
Other sizes were 205/75/15 and 225/75/15. Ground clearance decreases as does weight capacity. Other than that not much different.
BTW, 255/70/15's are roughly 29" tall and roughly 10" wide from sidewall to sidewall (not tread), 235/75/15's are roughly 28.875" tall and roughly 9.25" from sidewall to sidewall (not tread), 195/75/15" is 26.5" tall and 7.67" sidewall