Someone recently told me that stock pistons can be used in a 350 that has been bored .30 over as long as over sized rings are used. I dont quite know what to think of this. I always thought that over sized pistons were needed. Anyone have any insight into this?

You'll have to measure the pistons to see if they're larger than stock. If they're a stock type replacement piston for an overbore, there should be a .030" stamped on them somewhere. You can't use standard bore pistons in an overbored cylinder. There's always a specific piston to wall clearance specification when installing pistons.

I think you can clean up the cylinders to around .010" over and still use factory pistons with oversized rings but that may even be too much.

Someone gave you incorrect information,period.

You might could run stock pistons on a .010 over bored engine, I've ran a Briggs & Stratton lawnmower like that for years, it just rattles a little, but .030 would be out of the question, however if you bore the engine .30 over you wont have any cylinder walls left!!!