The last mentioned mod would be to make the bed compatible with a RSI smart cap or black diamond tonneau cover.
Judging by your reply this is either difficult to make both sides the same or compromises the structural integrity.
You do not need to make an Avalanche compatible with some aftermarket tonneau cover. It comes with a hard cover that is lockable.
I dont know how old you are or what your skill level is, but you mention GMC (?) being easy to work on. If you know that, you must have some idea that the undertaking you propose should be done by a very experienced professional. You need to be realistic. I dont believe there is a manual Avalanche it would be folly to try to alter the structural integrity of your vehicle unless you have immense experience. It is clear you dont. And that's ok. I'm not knocking you. Most people dont have that level of expertise.
But that just means you would spend more on alterations than you would to buy the vehicle, and then you wind up with a one off that cant easily be serviced