It will be fine to shoot the 158 gr in the 642. The 158 gr is my favorite 38 spl bullet weight. I've shot thousands of them and like the lead semi-waddcutter the best. They're great for paper target loads.
If that's all you can find buy and shoot them as they will be fine for you're S&W. The 158 gr round nose was the main round for the 38 spl for years and years.
NRA life member
When the going gets tough, I just open another beer.