What kind of Shower fitting can I install in a stock tank to tub conversion?
There are certainly bath tub drains that will work on thin metal tubs but often these have a stamped round depression around the drain hole that causes the drain top to be more level with the surrounding tub floor. This picture shows what this looks like from the side.



(Picture Source: https://edmondbathtubrefinishing.com/bathtub-plumbing-installation-drain-diagrams/)

Maybe this particular all metal type drain will be more suitable for your application. The top side flange of a metal drain will be thinner than a plastic one due to metal being stronger.



(Picture Source: https://www.amazon.com/KOHLER-K-9132-CP-Shower-Polished-Chrome/dp/B0014YLQE0)