So I'm pretty new to building engines, but I'd like to build my L31 engine up to have at least 350 hp. Will the stock 4L60-E tranny be able to handle that or do I need to upgrade?

 

Depends. It would be a very close call. Assuming that you are going to be punching it, I doubt a stock 4L60E is going to last very long. I would suggest an aftermarket
fluid/air cooling system if you keep the 4L60E.