How to "undo" cutting back of the cab

[Nova70]

First off, I will start with I am a complete audio nut so don't ask, "why?" please. lol.

Now, if I were to cut the back of the cab and the bed with a hole about 16" tall and roughly 28" wide or so, how would I go about reversing it in the future if I ever wanted to? Should I save the pieces I cut out and be able to weld them back in or have someone else weld them in? If I can do that, what should I use to cut the hole to keep the pieces in as good of shape as possible?

If you're wondering why I would cut a huge ass hole like that, I'm thinking about doing a blowthrough build for my stereo.