I bought a chassis almost a year ago and was finally planning to get started working on it. When I bought the car I knew about this damage and how it was repaired but I'm wondering if there is a better way to have gone about it. The guy said that he just welded in metal plates as you can see in the picture below and it help up perfectly fine aswell as passed inspection.

 

From personal experience, that is not repairable. That right there is your frame rail. Pulling that fram rail off is a major job and will cost a lot of cash. I'd HIGHLY recommend buying a new chassis, life is more important than trying to fix something like this.