IMHO, If I were going to weld the side skins in, I would go with thicker sheet metal. 18 or even 16ga. You'll get a lot less warping and easier to stich weld and grind down.
I went with 20 ga. and rivits because:
1. I'm working by myself
2. To weld, I would have to bring the jenny and welder to the storage yard, then take them home everynight.
3. My air snippers would not cut 16 ga.
4. It was the most covert way to skin the sides (My wife still don't know about this project
Note. I did not put rivets on one inch centers, just to much overkill.