Would shooting 1 1/4 oz. shotgun shells harm my shotgun?

I have only shot 1 oz shells before from it, but i was given some 1 1/4 (2 3/4 length, #8 shot). The gun is old, made probably around 1980. On the top of the barrel, it says "kmart cooperation model 151. proof tested, modified". Do you guys think it will be okay?