I had to do that to my rrp box on the outlet side. It was fairly easy, I did all the fab, cutting and mock up and then had it welded. Weld came out good and clean, box was also used. Can't remember the weld price though, prolly around $50.
At a retail welding shop the min charge is 1 hour, about $85 depending on the area. So figure $85 or less for the weld. You might want to get a local estimate though, with the fab work to help consume the min hour time span.
I cut a square around the pipe and removed it. got new plate larger than the square I cut out and hole sawed a new 2.5 inch hole n the new plate. Took new pipe in the shape I needed and slid it in the hole in the plate. Marked my locations and had the welder weld it.
In my case there is no retail box to buy, just had to mod it myself to get what I wanted
Not sure on yours
I should add, you'll need to study your box and make sure an inlet mod won't effect water back flow into your pipe or moving the Inlet pipe down wont run into a baffle. Also is removing the bottom of the box easier, thinking would be less work and almost no materials hmm....
